Neat gin? Surprisingly, it can stimulate an immune response (7)
I believe the answer is:
antigen
'it can stimulate an immune response' is the definition.
(antigens cause a response from the immune system)
'neat gin? surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters.
'neat'+'gin'='neatgin'
'neatgin' is an anagram of 'ANTIGEN'.
(Other definitions for antigen that I've seen before include "Biological substance" , "It stimulates immune responses - tag nine (anag)" , "Substance inducing an immune response" , "Immunity-booster" , "toxin" .)