Almost castrate setter (3)
I believe the answer is:
gel
'setter' is the definition.
(something which sets)
'almost castrate' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'castrate' becomes 'geld' (gelding is a kind of castrating).
'geld' with its final letter taken away is 'GEL'.
(Other definitions for gel that I've seen before include "Preparation used in setting hair" , "Semi-solid" , "Leg it and get it for your hair, say" , "posh lass" , "Come together, take shape (colloq.)" .)