Unpredictable grass invading Morecambe perhaps (7)
I believe the answer is:
erratic
'unpredictable' is the definition.
(synonyms)
'grass invading morecambe perhaps' is the wordplay.
'grass' becomes 'rat' (both can mean a snitch or informer).
'invading' is an insertion indicator.
'morecambe perhaps' becomes 'eric' (Eric Morecambe).
'rat' placed into 'eric' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Eccentric, aberrant" , "Wayward" , "Inconsistent, shifting" , "Not regular in pattern" , "Shifting, unpredictable" .)