Rowers have bird outside to raise (8)
I believe the answer is:
heighten
'to raise' is the definition.
(synonyms)
'rowers have bird outside' is the wordplay.
'rowers' becomes 'eight' (eight can mean a crew of eight rowers).
'have bird' becomes 'hen' (I am not sure about this - if you are sure you should believe this answer much more).
'outside' is an insertion indicator (some letters go outside others).
'eight' put into 'hen' is 'HEIGHTEN'.
Can you help me to learn more?
(Other definitions for heighten that I've seen before include "Intensify; make taller" , "Intensify, enhance" , "Become more intense" , "Lift" , "effect some improvement" .)