I had to go after cool drink in flat (7)
I believe the answer is:
insipid
'flat' is the definition.
(I know that flat can be written as insipid)
'i had to go after cool drink' is the wordplay.
'i had' becomes 'i'd' (I'd).
'to go after' says to put letters next to each other.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'drink' becomes 'sip' (sipping is a kind of drinking).
'in'+'sip'='insip'
'id' after 'insip' is 'INSIPID'.
'in' acts as a link.
(Other definitions for insipid that I've seen before include "Jejune" , "Dull" , "without taste" , "Colourless" , "Lacking 10" .)