Unpredictable boy catching rodent (7)
I believe the answer is:
erratic
'unpredictable' is the definition.
('erratic' can be a synonym of 'unpredictable')
'boy catching rodent' is the wordplay.
'boy' becomes 'eric' (I've seen this before**).
'catching' indicates putting letters inside.
'rodent' becomes 'rat' (rat is a kind of rodent).
'eric' placed around 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Uneven in movement or quality" , "Shifting, unpredictable" , "Irregular in motion or habit" , "Fluctuating" , "Irregular and unpredictable" .)