A fellow pocketing old penny as extra charge (3-2)
I believe the answer is:
add-on
'extra charge' is the definition.
(I've seen this before)
'a fellow pocketing old penny' is the subsidiary indication.
'fellow' becomes 'don' (I've seen this before).
'pocketing' is an insertion indicator.
'old penny' becomes 'd'.
'don' going around 'd' is 'ddon'.
'a'+'ddon'='addon'
'as' is the link.
(Other definitions for add-on that I've seen before include "Computer optional extra" , "Feature bolted on to a standard model or package" , "Extra bit" , "Feature supplementing standard model" , "Extra charge" .)