Understood fellow had one position to complete (2,3,8)
I believe the answer is:
go the distance
'complete' is the definition.
(to go the distance is to complete a boxing fight)
'understood fellow had one position' is the wordplay.
'understood' becomes 'got' (getting is a kind of understanding).
'fellow had' becomes 'he'd' (contraction of 'he had').
'one' becomes 'i' (Roman numeral).
'position' becomes 'stance' (stance is a kind of position).
'got'+'hed'+'i'+'stance'='GO THE DISTANCE'
'to' is the link.
(Other definitions for go the distance that I've seen before include "Complete a boxing match without being knocked out" , "finish off assignment" , "Complete job" , "Survive to end of contest" , "Stay to finish" .)