Charge double, initially, for forage (4)
I believe the answer is:
feed
'forage' is the definition.
(I've seen this before)
'charge double initially' is the wordplay.
'charge' becomes 'fee' (fee is a kind of charge**).
'initially' suggests taking the first letters.
The initial letter of 'double' is 'd'.
'fee'+'d'='FEED'
'for' acts as a link.
(Other definitions for feed that I've seen before include "Fodder" , "Nourish" , "take grub" , "Paid" , "forage" .)