By holding a payment back, is strong (5)
I believe the answer is:
beefy
'strong' is the definition.
(synonyms)
'by holding a payment back' is the wordplay.
'holding' indicates putting letters inside.
'a payment' becomes 'fee' (I've seen this before).
'back' says the letters should be written in reverse.
'fee' back-to-front is 'eef'.
'by' going around 'eef' is 'BEEFY'.
'is' acts as a link.
(Other definitions for beefy that I've seen before include "Burly" , "like some bully?" , "Strong and muscular" , "Solid" , "Colloquially muscular" .)