Light headed, always, in the bar (5)
I believe the answer is:
lever
'bar' is the definition.
(lever is a kind of bar)
'light headed always in' is the wordplay.
'light' becomes 'uv' (UV or ultraviolet light).
'headed' means to remove the first letter (head can mean to remove the top from something).
'always' becomes 'leer' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'uv' with its first letter taken away is 'v'.
'v' going into 'leer' is 'LEVER'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for lever that I've seen before include "Heavy bar" , "Lifting bar pivoted on a fulcrum" , "Means of exerting force" , "Jemmy" , "Rigid bar pivoting about a fulcrum" .)