Around fifty, one begins to struggle (8)
I believe the answer is:
flounder
'struggle' is the definition.
(flounder can mean to struggle)
'around fifty one begins to' is the wordplay.
'around' indicates putting letters inside.
'fifty' becomes 'l' (Roman numeral).
'begins' suggests taking the first letters.
'to' becomes 'funder' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'one' is 'o'.
'l'+'o'='lo'
'lo' inserted into 'funder' is 'FLOUNDER'.
Can you help me to learn more?
(Other definitions for flounder that I've seen before include "A fish; be out of depth" , "Predatory fish" , "Struggle clumsily - flatfish" , "small swimmer" , "fumble?" .)