Extent of special study (6)
I believe the answer is:
spread
'extent' is the definition.
(I've seen this before)
'special study' is the wordplay.
'special' becomes 'sp' (abbreviation).
'study' becomes 'read' (synonyms).
'sp'+'read'='SPREAD'
'of' is the link.
(Other definitions for spread that I've seen before include "Distribute or disperse widely" , "Type of betting" , "Broadcast" , "Open out; range" , "Gradually reach a wider area" .)