I’ll do that which is largely counted on at the outset (5)
I believe the answer is:
wilco
'i'll do that' is the definition.
(military term derived from 'will comply')
'which is largely counted on at the outset' is the wordplay.
'at the outset' says to take the initial letters.
The first letters of 'which is largely counted on' is 'WILCO'.
(Other definitions for wilco that I've seen before include "Roger might be followed by this" , "Radio comms expression of compliance" , "expression of agreement" , "Agreed (radio comms)" , "it indicated acquiescence" .)