Long Island invested in fundamental American church (8)
I believe the answer is:
basilica
'church' is the definition.
(basilica is a kind of church)
'long island invested in fundamental american' is the wordplay.
'long' becomes 'l'.
'island' becomes 'i'.
'invested in' is an insertion indicator.
'fundamental' becomes 'basic' ('basic' can be similar in meaning to 'fundamental'**).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'l'+'i'='li'
'li' placed inside 'basic' is 'basilic'.
'basilic'+'a'='BASILICA'
(Other definitions for basilica that I've seen before include "Important Roman Catholic church" , "Bails CIA out of the large church" , "Important church, like St Peter's in Rome" , "Roman law court; church" , "Romanesque church" .)