Drink popular ale, ultimately it's flat (6)
I believe the answer is:
supine
'flat' is the definition.
(thesaurus)
'drink popular ale ultimately' is the wordplay.
'drink' becomes 'sup' (I've seen this before**).
'popular' becomes 'in'.
'ultimately' says to take the final letters.
The final letter of 'ale' is 'e'.
'sup'+'in'+'e'='SUPINE'
'it's' acts as a link.
(Other definitions for supine that I've seen before include "Laid-back" , "Use pin (anag.)" , "offering no resistance" , "looking up" , "Unresisting" .)