Once strain, creating annexe? (9)
I believe the answer is:
extension
'annexe?' is the definition.
('extension' can be a synonym of 'annexe')
'once strain' is the wordplay.
'once' becomes 'ex'.
'strain' becomes 'tension' ('tension' can be a synonym of 'strain').
'ex'+'tension'='EXTENSION'
'creating' is the link.
(Other definitions for extension that I've seen before include "more rooms in house maybe" , "telephone number" , "Office phone, perhaps" , "Another phone" , "Additional time for repayment" .)