Bound to be a dollar in month's returns (5)
I believe the answer is:
cased
'bound' is the definition.
(similar in meaning)
'a dollar in month's returns' is the wordplay.
'dollar' becomes 's'.
'in' indicates putting letters inside.
'month' becomes 'dec' (abbreviation for December).
'returns' is a reversal indicator.
'dec' backwards is 'ced'.
'a'+'s'='as'
'as' put within 'ced' is 'CASED'.
'to be' acts as a link.
(Other definitions for cased that I've seen before include "Carefully inspected (for crime)" , "Reconnoitred (sl.)" , "examined with intention to rob" , "put in a box" , "Covered, protected; checked over (by robber)" .)