Grayling is giving up heroin in great trouble (6)
I believe the answer is:
crisis
'great trouble' is the definition.
'grayling is giving up heroin' is the wordplay.
'grayling' becomes 'chris' (politician Chris Grayling).
'giving up' indicates named letters should be taken away.
'heroin' becomes 'h' (synonyms).
'chris'+'is'='chrisis'
'chrisis' with 'h' taken out is 'CRISIS'.
'in' is the link.
(Other definitions for crisis that I've seen before include "Climax or turning point as in fever" , "Decisive time, breaking point" , "unstable period" , "Time of intense difficulty or danger" , "Decisive moment, turning point" .)