Unpredictable boy sheltering rodent (7)
I believe the answer is:
erratic
'unpredictable' is the definition.
(synonyms)
'boy sheltering rodent' is the wordplay.
'boy' becomes 'eric' (I've seen this before).
'sheltering' means one lot of letters goes inside another.
'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 "Shifting, unpredictable" , "Volatile" , "Behaving irregularly" , "Wild" , "Changeable" .)