Setter's leg-over? (3)
I believe the answer is:
gel
'setter's' is the definition.
(something which sets)
'leg-over?' is the wordplay.
'over?' is a reversal indicator (I've seen 'over ' mean this).
'leg' reversed gives 'GEL'.
(Other definitions for gel that I've seen before include "It may be applied to locks" , "Semi-solid" , "Take form" , "Leg it back to get some for your hair" , "Congeal - hair dressing" .)