Deposit concerning banks in foolhardy decay (7)
I believe the answer is:
putrefy
'decay' is the definition.
(I know that putrefy is a more specific form of decay)
'deposit concerning banks in foolhardy' is the wordplay.
'deposit' becomes 'put' (I've seen this in another clue).
'concerning' becomes 're' (both can mean on a particular topic).
'banks in' means to remove the middle letters (the edges or banks of the word).
'foolhardy' with its middle taken out is 'fy'.
'put'+'re'+'fy'='PUTREFY'
(Other definitions for putrefy that I've seen before include "Decay badly" , "Go off" , "Decay, rot" , "Corrupt" , "Decay fetidly" .)