In armed force, gunners are at the forefront (3)
I believe the answer is:
raf
'in armed force' is the definition.
'raf' can be an answer for 'force'. I'm not sure about the 'in armed' bit.
'gunners are at the forefront' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'are at' means one lot of letters go next to another.
'the forefront' becomes 'f' (this might be a standard abbreviation which I don't know about).
'ra'+'f'='RAF'
Can you help me to learn more?
(Other definitions for raf that I've seen before include "It fought Battle of Britain" , "Royal Air Force (abbr.)" , "Airforce of Great Britain" .)