Idle maybe, clutching rodent? Most unreliable (7)
I believe the answer is:
erratic
'most unreliable' is the definition.
(something erratic is very unreliable)
'idle maybe clutching rodent?' is the wordplay.
'idle maybe' becomes 'eric' (Eric Idle).
'clutching' is an insertion indicator.
'rodent?' becomes 'rat' (rat is a kind of rodent).
'eric' enclosing 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Irregular and unreliable" , "Uneven in movement or quality" , "Random, unreliable" , "Unpredictable; occasional" , "Irregular, not dependable" .)