Capable of leading time -- with oxygen! (2,2)
I believe the answer is:
up to
'capable of' is the definition.
(I've seen this before)
'leading time with oxygen' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'time' becomes 't'.
'with' means one lot of letters go next to another.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'up'+'t'+'o'='UP TO'
(Other definitions for up to that I've seen before include "As far as or as many as" , "As many as, though no more" , "Busy with" , "As far as a certain point" , "Capable of" .)