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 am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'uv' with its first letter removed is 'v'.
'v' put within 'leer' is 'LEVER'.
'the' is the 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" .)