Thrash about and miss the mark outside 50 (5)
I believe the answer is:
flail
'thrash about' is the definition.
(I've seen this before)
'miss the mark outside 50' is the wordplay.
'miss the mark' becomes 'fail' (I've seen this in another clue).
'outside' is an insertion indicator (some letters go outside others).
'50' becomes 'l' (Roman numeral).
'fail' enclosing 'l' is 'FLAIL'.
'and' acts as a link.
(Other definitions for flail that I've seen before include "Wave about wildly" , "Thresher; wave helplessly about" , "thrash around" , "Threshing implement" , "Old tool for threshing grain" .)