Know-it-all lives in care we suspect (8)
I believe the answer is:
wiseacre
'know-it-all' is the definition.
(I've seen this before)
'lives in care we suspect' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'in' indicates putting letters inside.
'suspect' indicates an anagram.
'care'+'we'='carewe'
'carewe' anagrammed gives 'weacre'.
'is' placed inside 'weacre' is 'WISEACRE'.
(Other definitions for wiseacre that I've seen before include "Person with an affectation of wisdom" , "Smart alec" , "Smart Aleck" , "Person who tries to appear clever" , "Self-appointed sage" .)