One always behind Labour lass on the rise (7)
I believe the answer is:
laggard
'one' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I don't see?
'always behind labour lass on the rise' is the wordplay.
'always behind labour' becomes 'drag' (I am not sure about this - if you are sure you should believe this answer much more).
'lass' becomes 'gal' (both can mean a girl).
'on the rise' says the letters should be written in reverse (in a down clue, letters go up).
'drag'+'gal'='draggal'
'draggal' in reverse letter order is 'LAGGARD'.
Can you help me to learn more?
(Other definitions for laggard that I've seen before include "slowcoach" , "One failing to keep up" , "Moving slowly" , "He drags along behind" , "one going slowly" .)