Mercurial man protects informer (7)
I believe the answer is:
erratic
'mercurial' is the definition.
(I know that mercurial can be written as erratic)
'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' going around 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Irregular or uneven, not steady" , "Cartier (anag.)" , "hit-and-miss" , "Liable to sudden unpredictable change" , "Hit-or-miss" .)