Old enough to start earning immediately (4)
I believe the answer is:
enow
'old enough' is the definition.
(I've seen this in another clue)
'start earning immediately' is the wordplay.
'start' indicates taking the first letters.
'immediately' becomes 'now' (I've seen this in another clue).
The first letter of 'earning' is 'e'.
'e'+'now'='ENOW'
'to' acts as a link.
(Other definitions for enow that I've seen before include "adequate" , "Sufficient in the past" , "a moment ago" , "Old enough" , "Enough (archaic)" .)