See rust as the hard outer part (5)
I believe the answer is:
crust
'the hard outer part' is the definition.
'crust' can be an answer for 'part' (crust is a kind of part). I am not certain of the 'the hard outer' bit.
'see rust' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'c'+'rust'='CRUST'
'as' is the link.
Can you help me to learn more?
(Other definitions for crust that I've seen before include "Tough part of twenty two down" , "Hard outer coating" , "Bit of toast, perhaps" , "Earth's outer layer" , "Pie covering" .)