A small drink mostly ale, that's all (5)
I believe the answer is:
total
'all' is the definition.
(thesaurus)
'a small drink mostly ale' is the wordplay.
'a small drink' becomes 'tot' (I've seen this before).
'mostly' means to remove the last letter.
'ale' with its final letter taken off is 'al'.
'tot'+'al'='TOTAL'
'that's' is the link.
(Other definitions for total that I've seen before include "Including all" , "Complete; add up" , "It might be grand" , "Full, entire amount" , "Completely destroy" .)