Long for oxygen during hard exercise (4)
I believe the answer is:
hope
'long for' is the definition.
(thesaurus)
'oxygen during hard exercise' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'during' means one lot of letters goes inside another.
'hard' becomes 'h'.
'exercise' becomes 'pe' (Physical Education).
'h'+'pe'='hpe'
'o' inserted into 'hpe' is 'HOPE'.
(Other definitions for hope that I've seen before include "virtue he needs?" , "Trustful expectation" , "English novelist" , "Expect and wish" , "The Cape of Good -" .)