A masculine moustache resembling shells (4)
I believe the answer is:
ammo
'shells' is the definition.
(I've seen this before)
'a masculine moustache' is the wordplay.
'masculine' becomes 'm'.
'moustache' becomes 'mo' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'m'+'mo'='AMMO'
'resembling' acts as a link.
Can you help me to learn more?
(Other definitions for ammo that I've seen before include "Bullets etc.(informal)" , "rounds briefly" , "Bullets, shells (colloq.)" , "Bullets, shells, rockets etc" , "Bullet" .)