Visitor putting energy into blow (5)
I believe the answer is:
guest
'visitor' is the definition.
(guest is a kind of visitor)
'energy into blow' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mcu).
'into' indicates putting letters inside.
'blow' becomes 'gust' ('gust' can be a synonym of 'blow'**).
'e' placed inside 'gust' is 'GUEST'.
'putting' acts as a link.
(Other definitions for guest that I've seen before include "One who's invited" , "Someone invited" , "Person receiving another's hospitality" , "Caller" , "Invitee" .)