Gunman originally uses rocks for shot (5)
I believe the answer is:
guess
'shot' is the definition.
(synonyms)
'gunman originally uses rocks' is the wordplay.
'originally' suggests taking the first letters.
'rocks' indicates an anagram.
The initial letter of 'gunman' is 'g'.
'uses' anagrammed gives 'uess'.
'g'+'uess'='GUESS'
'for' acts as a link.
(Other definitions for guess that I've seen before include "Estimate on the basis of little information" , "shot?" , "Theory" , "Estimate roughly" , "Make stab at" .)