Suffering or understanding (6)
I believe the answer is:
ordeal
'suffering' is the definition.
(I've seen this before)
'or understanding' is the wordplay.
'understanding' becomes 'deal' (deal is a kind of understanding).
'or'+'deal'='ORDEAL'
(Other definitions for ordeal that I've seen before include "Torment" , "Trouble" , "Severly trying experience" , "Torture" , "Severe or trying experience" .)