Not to mention claw found in shelter (3,5)
I believe the answer is:
let alone
'not to mention' is the definition.
('let alone' can be a synonym of 'not to mention')
'claw found in shelter' is the wordplay.
'claw' becomes 'talon' (talon is a kind of claw).
'found in' means one lot of letters goes inside another.
'shelter' becomes 'lee' (I've seen this before).
'talon' going inside 'lee' is 'LET-ALONE'.
(Other definitions for let alone that I've seen before include "Don't touch" , "Don't bother someone - allow to be solitary" , "Not to mention, never mind" , "Not to mention - much less - avoid" , "Not to mention (thing)" .)