I sprint, keeping on? I wish (2,4)
I believe the answer is:
if only
'i wish' is the definition.
(I've seen this before)
'i sprint keeping on?' is the wordplay.
'sprint' becomes 'fly' (I've seen this in another clue).
'keeping' is an insertion indicator (some letters kept inside).
'fly' placed around 'on' is 'fonly'.
'i'+'fonly'='IF ONLY'
(Other definitions for if only that I've seen before include "Would that" , "I wish that could be provided just after" , "in my dreams" , "expression of wish" .)