US gunman is surprisingly modest (10)
I believe the answer is:
unassuming
'modest' is the definition.
(I've seen this before)
'us gunman is surprisingly' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'us'+'gunman'+'is'='usgunmanis'
'usgunmanis' anagrammed gives 'UNASSUMING'.
(Other definitions for unassuming that I've seen before include "disrobing?" , "not taking things for granted" , "Modest" , "Not pretentious" .)