Crop: staple source of sustenance, on reflection (4)
I believe the answer is:
snip
'crop' is the definition.
(synonyms)
'staple source of sustenance on reflection' is the wordplay.
'staple' becomes 'pin'.
'source of' suggests taking the first letters.
'on reflection' shows that the letters should be reversed in order.
The first letter of 'sustenance' is 's'.
'pin'+'s'='pins'
'pins' back-to-front is 'SNIP'.
(Other definitions for snip that I've seen before include "Something cheap or easy to do" , "Bargain (coll)" , "nick)" , "Cut with shears" , "Surprisingly cheap item" .)