Weak part of racket effective on return (6)
I believe the answer is:
effete
'weak' is the definition.
(I've seen this in another clue)
'part of racket effective on return' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'on return' says the letters should be written in reverse.
'eteffe' can be found hidden inside 'racket effective'.
'eteffe' back-to-front is 'EFFETE'.
(Other definitions for effete that I've seen before include "Ineffective, decadent" , "Feeble, having lost vitality" , "Lacking in power" , "Over-refined; weak" , "Affected, ineffectual" .)