About to imbibe first of gins on the rocks (7)
I believe the answer is:
aground
'on the rocks' is the definition.
(I've seen this before)
'about to imbibe first of gins' is the wordplay.
'about' becomes 'around' (synonyms).
'to imbibe' indicates putting letters inside (to imbibe can mean to drink or absorb).
'first of gins' becomes 'g' (1st letter of 'gins').
'around' placed around 'g' is 'AGROUND'.
(Other definitions for aground that I've seen before include "No longer floating" , "(Of a ship) stuck on the seabed" , "(Of ship) not floating" , "Beached" , "out of water" .)