Reduced weight with badly needed determination (4)
I believe the answer is:
will
'determination' is the definition.
(thesaurus)
'reduced weight with badly' is the wordplay.
'reduced' says to take the initial letters (the word is reduced to its first letter).
'with' means one lot of letters go next to another.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
The initial letter of 'weight' is 'w'.
'w'+'ill'='WILL'
'needed' is the link.
(Other definitions for will that I've seen before include "Last testament" , "Written directions for disposal of estate after death" , "Final disposition" , "- And Grace, TV series" , "Intention; bequeath" .)