can a model take aim while engage

[bold yarrow]

If a model declares take aim and has dual attack but is engaged could he melee kill models that is engaged with and then shoot

[left kettle]

Yes but only with initials. You can’t buy attacks until you are done with initials.
If you clear all the models engaging you then you can shoot with the aim bonus regardless of whether you are engaging other models.

[calm acorn]

If you kill enemy models which engage your model you will have aim bonus.

[stark swallow]

You do not gain the bonus from Aiming while engaged, but you can still aim. Since Aiming is your normal movement (not something for which you forfeit your normal movement, a common misconception) you can also Bulldoze anyone that is B2B with you by Aiming. If that pushes them out of engagement (or if you kill them with melee initials and have Dual Attack) then your ranged shots will get the Aim bonus.

Basically you're allowed to aim always, but if at the moment you shoot you are currently engaged, you miss out on the bonus.

Another example is aiming and shooting with a weapon with beatback and multiple shots (like the Dozer gun). If you push the enemy so it's no longer engaging you, you will get the aim bonus on subesquent shots.