They are something to build on (5)
I believe the answer is:
sites
'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps you can see a link between them that I can't see?
'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'.
'are' acts as a link.
Can you help me to learn more?
(Other definitions for sites that I've seen before include "Spots" , "Is set (anag)" , "places" , "internet locations" , "Is set among building locations" .)