The rigour of a difficult craft (8)
I believe the answer is:
hardship
'the rigour' is the definition.
('hardship' can be a synonym of 'rigour')
'a difficult craft' is the wordplay.
'a difficult' becomes 'hard' (I've seen this before).
'craft' becomes 'ship' (ship is a kind of craft).
'hard'+'ship'='HARDSHIP'
'of' is the link.
(Other definitions for hardship that I've seen before include "distress" , "Severe difficulty or suffering" , "Difficulty, vicissitude" , "Austerity - affliction" , "Adversity, misfortune" .)