Unreliable man protects informer (7)
I believe the answer is:
erratic
'unreliable' is the definition.
(similar in meaning)
'man protects informer' is the wordplay.
'man' becomes 'eric' (I've seen this before).
'protects' is an insertion indicator (outer letters protect inside letters).
'informer' becomes 'rat' (synonyms).
'eric' placed around 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Unpredictable - car tire (anag)" , "Shifting, unpredictable" , "Irregular or uneven in movement or habits" , "Liable to sudden unpredictable change" , "Irregular in motion or habit" .)