Before short trek, make up a posh liqueur (9)
I believe the answer is:
cointreau
'liqueur' is the definition.
(I've seen this in another clue)
'before short trek make up a posh' is the wordplay.
'before' means one lot of letters go next to another.
'short' means to remove the last letter.
'make up' becomes 'coin' (I've seen this before).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'trek' with its final letter taken away is 'tre'.
'tre' put after 'coin' is 'cointre'.
'cointre'+'a'+'u'='COINTREAU'
(Other definitions for cointreau that I've seen before include "Sweet orange-flavoured colourless liqueur" , "Alcoholic drink" , "A sweet orange-flavoured liqueur for cautioner" , "Orange liqueur" .)