For instance, in building sites, one can find a writer! (8)
I believe the answer is:
essayist
'writer' is the definition.
(I know that essayist is a type of writer)
'for instance in building sites' is the wordplay.
'for instance' becomes 'say' (both can be used to introduce an example).
'in' is an insertion indicator.
'building' is an anagram indicator.
'sites' with letters rearranged gives 'esist'.
'say' going into 'esist' is 'ESSAYIST'.
'one can find a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for essayist that I've seen before include "Montaigne, say" , "Bacon maybe" , "Writer (e.g. Lamb)" , "Writer of pieces on various subjects" , "The venturer" .)