Left also goes ahead and fights manual workers' cases (9)

I believe the answer is:

toolboxes

'manual workers' cases' is the definition.

'left also goes ahead and fights' is the wordplay.
'left' becomes 'l' (common abbreviation).
'also' becomes 'too' ('too' can be a synonym of 'also').
'goes ahead' is a charade indicator (letters next to each other).
'and' says to put letters next to each other.
'fights' becomes 'boxes'.
'l' put after 'too' is 'tool'.
'tool'+'boxes'='TOOLBOXES'

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app