American, leader of team leading early on (2,5)
I believe the answer is:
at first
'early on' is the definition.
(at first can mean originally)
'american leader of team leading' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'leader of' says to take the initial letters.
'leading' becomes 'first' (I've seen this before).
The first letter of 'team' is 't'.
'a'+'t'+'first'='AT FIRST'
(Other definitions for at first that I've seen before include "Originally" , "In the beginning, initially" , "To begin with" , "In the beginning, to start with" .)