Not shrewd? That's strange (7)
I believe the answer is:
uncanny
'that's strange' is the definition.
(I've seen this before)
'not shrewd?' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'shrewd?' becomes 'canny' (both can mean having good judgement).
'un'+'canny'='UNCANNY'
(Other definitions for uncanny that I've seen before include "Weird - strange" , "Weird, unearthly" , "Weird, unnatural" , "Strange, beyond the normal" , "Incredible" .)