Agent Philby's entered, somewhat thin (6)
I believe the answer is:
skimpy
'somewhat thin' is the definition.
'skimpy' can be an answer for 'thin' (thesaurus). I'm not sure about the 'somewhat' bit.
'agent philby's entered' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'philby' becomes 'kim' (spy Kim Philby).
'entered' indicates putting letters inside.
'spy' enclosing 'kim' is 'SKIMPY'.
Can you help me to learn more?
(Other definitions for skimpy that I've seen before include "Meagre; (of clothes) short and revealing" , "'Scanty, barely enough (6)'" , "Containing little excess" , "Providing little coverage" , "Inadequate (of clothing)" .)