It's most wise to decline drugs on start of tour (6)
I believe the answer is:
sagest
'it's most wise' is the definition.
('sage' can be similar in meaning to 'wise')
'decline drugs on start of tour' is the wordplay.
'decline' becomes 'sag' (I've seen this before).
'drugs' becomes 'es' (E is a term for an ecstasy pill).
'on' says to put letters next to each other.
'start of' suggests taking the first letters.
The initial letter of 'tour' is 't'.
'sag'+'es'+'t'='SAGEST'
'to' is the link.
(Other definitions for sagest that I've seen before include "Gas set (anag.)" , "Wisest" , "Extremely clever" , "Most discerning" , "Most wise and prudent" .)