An inch? I’ll say! (4)
I believe the answer is:
isle
'an inch?' is the definition.
(I've seen this before)
'i'll say' is the wordplay.
'say' shows a homophone (sound like).
'ill' sounds like 'ISLE'.
(Other definitions for isle that I've seen before include "eg Thanet" , "Small mass of land in the sea" , "Detached territory" , "Capri for one" , "being impossible without water" .)