Posh female set (3)
I believe the answer is:
gel
I believe this clue is a double definition.
'posh female' is the first definition.
(I've seen this before)
'set' is the second definition.
(both can mean to harden)
(Other definitions for gel that I've seen before include "stiffen" , "Hair styling substance" , "Set, become firmer" , "Semi-solid" , "Solidify" .)