Bunk, I said, used to be a little hot (7)
I believe the answer is:
eyewash
'bunk' is the definition.
(I've seen this before)
'i said used to be a little hot' is the wordplay.
'said' shows a homophone (sound like).
'used to be' becomes 'was' (I've seen this before).
'a little' indicates taking the first letters.
The first letter of 'hot' is 'h'.
'i' sounds like 'eye'.
'eye'+'was'+'h'='EYEWASH'
(Other definitions for eyewash that I've seen before include "Nonsense (colloq.)" , "rot?" , "Optic solution to relieve irritation - rubbish!" , "Rhubarb" , "Pupils may benefit from this" .)