Prepare to fire volunteer one's seen holding parrot (4,3)
I believe the answer is:
take aim
'prepare to fire' is the definition.
(I've seen this in another clue)
'volunteer one's seen holding parrot' is the wordplay.
'volunteer' becomes 'TA' (member of the Territorial Army, volunteer regiment).
'one's seen' becomes 'im' (I am not sure about this - if you are sure you should believe this answer much more).
'holding' means one lot of letters goes inside another.
'parrot' becomes 'kea' (kea is a kind of parrot).
'ta'+'im'='taim'
'taim' enclosing 'kea' is 'TAKE AIM'.
Can you help me to learn more?
(Other definitions for take aim that I've seen before include "train" , "Penultimate order for those shooting" , "Point gun at one's target" , "Line up weapon" , "Sight a target" .)