Struggle to pass beneath, having lost weight (8)
I believe the answer is:
flounder
'struggle' is the definition.
(flounder can mean to struggle)
'to pass beneath having lost weight' is the wordplay.
'to pass' means one lot of letters go next to another.
'beneath' becomes 'under' (under means beneath or underneath).
'having lost weight' becomes 'flo' (I can't justify this - if you can you should give a lot more credence to this answer).
'under' put after 'flo' is 'FLOUNDER'.
Can you help me to learn more?
(Other definitions for flounder that I've seen before include "small swimmer" , "Catch" , "Fish; stagger clumsily" , "Predatory fish" , "Be out of one's depth" .)