Duke to have a go in decline (8)
I believe the answer is:
downturn
'decline' is the definition.
(I've seen this before)
'duke to have a go' is the wordplay.
'duke' becomes 'd' (abbreviation).
'to have' becomes 'own' (synonyms).
'a go' becomes 'turn' ('turn' can be a synonym of 'go').
'd'+'own'+'turn'='DOWNTURN'
'in' acts as a link.
(Other definitions for downturn that I've seen before include "Deterioration" , "Worsening of economic activity" , "Reduction in business" , "Decline in business" , "Decline eg of economic activity" .)