One perhaps choosing Green carrot, celery sandwiches when retired (7)
I believe the answer is:
elector
'one perhaps choosing green' is the definition.
I can't tell whether this definition defines the answer.
'carrot celery sandwiches when retired' is the wordplay.
'sandwiches' says the answer is hidden in the clue (the answer is sandwiched in by the clue).
'when retired' is a reversal indicator (retire can mean to go back).
'rotcele' is hidden in the letters of 'carrot celery'.
'rotcele' in reverse letter order is 'ELECTOR'.
Can you help me to learn more?
(Other definitions for elector that I've seen before include "One may make one cross" , "A prince of the Holy Roman Empire" , "Crosses" , "Someone with the right to vote" , "one with an X" .)