Wealth of men heartlessly collecting shells (6)
I believe the answer is:
mammon
'wealth' is the definition.
(mammon is a kind of wealth)
'men heartlessly collecting shells' is the wordplay.
'heartlessly' suggests removing the centre.
'collecting' indicates putting letters inside.
'shells' becomes 'ammo' (I've seen this before).
'men' with its middle removed is 'mn'.
'mn' going around 'ammo' is 'MAMMON'.
'of' acts as a link.
(Other definitions for mammon that I've seen before include "Greed personified" , "Wealth (worshipped)" , "Bible 'You cannot serve God and . . . . . .'" , "God of material wealth" , "Wealth personified, regarded as evil influence" .)