Understand growth in suffering (9)
I believe the answer is:
heartache
'suffering' is the definition.
(I've seen this before)
'understand growth' is the wordplay.
'understand' becomes 'hear' (hear can mean to acknowledge or understand).
'growth' becomes 'tache' (short for moustache).
'hear'+'tache'='HEARTACHE'
'in' acts as a link.
(Other definitions for heartache that I've seen before include "Great sadness" , "Extreme emotional pain" , "Agony of Love" , "expression of sorrow" , "Suffering" .)