Learnt to work around oxygen afterwards (5,2)
I believe the answer is:
later on
'afterwards' is the definition.
(synonyms)
'learnt to work around oxygen' is the wordplay.
'to work' indicates an anagram (letters worked into a new order).
'around' is an insertion indicator.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'learnt' anagrammed gives 'latern'.
'latern' going around 'o' is 'LATER ON'.
(Other definitions for later on that I've seen before include "In the fullness of time" , "this evening?" , "Afterwards" , "in the future" , "'Subsequently, afterward (5,2)'" .)