Extending study, one in annexe? (8)
I believe the answer is:
widening
'extending' is the definition.
('widen' can be a synonym of 'extend')
'study one in annexe?' is the wordplay.
'study' becomes 'den' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'annexe?' becomes 'wing' ('wing' can be a synonym of 'annexe').
'den'+'i'='deni'
'deni' placed within 'wing' is 'WIDENING'.
(Other definitions for widening that I've seen before include "Extending" , "Increasing size" , "increasing girth" , "Broadening" .)