The bar on the left, always (5)
I believe the answer is:
lever
'the bar' is the definition.
(lever is a kind of bar)
'on the left always' is the wordplay.
'on' means one lot of letters go next to another.
'the left' becomes 'L' (common abbreviation).
'always' becomes 'ever' (synonyms).
'l' put next to 'ever' is 'LEVER'.
(Other definitions for lever that I've seen before include "exert force, in a way" , "Bar for prising something" , "Pivoting bar, handle" , "Lifting bar pivoted on a fulcrum" , "Machine operator" .)