Dependent on a set of books about Long Island (7)
I believe the answer is:
reliant
'dependent' is the definition.
(I've seen this before)
'on a set of books about long island' is the wordplay.
'on' says to put letters next to each other.
'set of books' becomes 'nt' (New Testament).
'about' becomes 're' (common abbreviation for 'with reference to').
'long' becomes 'l'.
'island' becomes 'i'.
'a'+'nt'='ant'
're'+'l'+'i'='reli'
'ant' put after 'reli' is 'RELIANT'.
(Other definitions for reliant that I've seen before include "car firm" , "Dependent - latrine (anag)" , "Robin was one < in need of support" , "leaning" , "Dependent upon" .)