Forking out pence in mailing? (8)
I believe the answer is:
spending
'forking out' is the definition.
(to fork out is to spend a lot of money)
'pence in mailing?' is the wordplay.
'pence' becomes 'p'.
'in' indicates putting letters inside.
'mailing?' becomes 'sending' ('send' can be a synonym of 'mail').
'p' inserted into 'sending' is 'SPENDING'.
(Other definitions for spending that I've seen before include "'''Getting and ..., we lay waste our powers'' (Wordsworth) (8)'" , "Laying out time or money" , "expenses" , "Using money or passing time" , "splashing out?" .)