Cool drink setter had in flat (7)
I believe the answer is:
insipid
'flat' is the definition.
('insipid' can be a synonym of 'flat')
'cool drink setter had' is the wordplay.
'cool' becomes 'in' (I've seen this in other clues).
'drink' becomes 'sip' (sipping is a kind of drinking**).
'setter' becomes 'i' (the person that set the crossword).
'had' becomes 'd' (I've seen this in other clues).
'in'+'sip'+'i'+'d'='INSIPID'
'in' is the link.
(Other definitions for insipid that I've seen before include "Colourless" , "Bland, flavourless" , "dry" , "Jejune" , "without taste" .)