They're something to build on (5)
I believe the answer is:
sites
'they're' is the definition.
The definition suggests an adverb but the answer is not.
'something to build on' is the wordplay.
'something' becomes 'it' (I've seen this in other clues).
'to build' becomes 'ses' (I can't justify this - if you can you should believe this answer much more).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'it' placed into 'ses' is 'SITES'.
Can you help me to learn more?
(Other definitions for sites that I've seen before include "Locates" , "places" , "Spots" , "Locations or positions, as for building" , "Is set among building locations" .)