Deadhead gets poor returns to sag (5)
I believe the answer is:
droop
'to sag' is the definition.
(synonyms)
'deadhead gets poor returns' is the wordplay.
'deadhead' becomes 'd' (head letter of 'dead').
'gets' says to put letters next to each other.
'returns' is a reversal indicator.
'poor' backwards is 'roop'.
'd'+'roop'='DROOP'
(Other definitions for droop that I've seen before include "Sag wearily" , "Go limp" , "Hang downwards, lose strength or spirit" , "male problem?" , "languish" .)