... secret drinker seen in almost sinister clubs (8)
I believe the answer is:
esoteric
'secret' is the definition.
('esoteric' can be similar in meaning to 'secret')
'drinker seen in almost sinister clubs' is the wordplay.
'drinker' becomes 'sot' (sot is a kind of drinker**).
'seen in' is an insertion indicator.
'almost' means to remove the last letter (most of the word but not all of it).
'sinister' becomes 'eerie' (I've seen this before).
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'eerie' with its last letter taken off is 'eeri'.
'sot' placed into 'eeri' is 'esoteri'.
'esoteri'+'c'='ESOTERIC'
(Other definitions for esoteric that I've seen before include "Intelligible only with special knowledge" , "Privileged or confidential among coteries" , "Secret, mysterious" , "Highly specialised" , "Inner - secret" .)