Fly high? Sounds painful! (4)
I believe the answer is:
soar
'fly high?' is the definition.
(I've seen this before)
'sounds painful' is the wordplay.
'sounds' shows a homophone (sound like).
'painful' becomes 'sore' ('sore' can be a synonym of 'painful').
'sore' is a homophone of 'SOAR'.
(Other definitions for soar that I've seen before include "Rise effortlessly" , "go high" , "Rise high in flight" , "Fly high into the air" , "Fly high, Leicester river" .)