Tie up fifty-one (4)
I believe the answer is:
lace
'tie up' is the definition.
(thesaurus)
'fifty-one' is the wordplay.
'fifty' becomes 'l' (roman numeral).
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'l'+'ace'='LACE'
(Other definitions for lace that I've seen before include "Fine decorative fabric" , "Tear" , "Shoe cord" , "Delicate open-weave fabric" , "Do up (shoe, etc)" .)