Relaxing one's guard is risky (9)
I believe the answer is:
dangerous
'risky' is the definition.
('dangerous' can be similar in meaning to 'risky')
'relaxing one's guard' is the wordplay.
'relaxing' indicates an anagram.
'ones'+'guard'='onesguard'
'onesguard' with letters rearranged gives 'DANGEROUS'.
'is' is the link.
(Other definitions for dangerous that I've seen before include "menacing" , "Hazardous" , "Unsafe, risky" , "Pope: 'A little learning is a . . . . . . . . . thing'" , "Hairy" .)