Branching out, this writer had taken up poetry (15)
I believe the answer is:
diversification
'branching out' is the definition.
'this writer had taken up poetry' is the wordplay.
'this writer had' becomes 'id' ('I'd' refers to the person writing the clues).
'taken up' shows that the letters should be reversed in order.
'poetry' becomes 'versification'.
'id' backwards is 'di'.
'di'+'versification'='DIVERSIFICATION'
(Other definitions for diversification that I've seen before include "Making more variety" , "Process of widening one's range of activities" .)