51 in company of Muse (4)
I believe the answer is:
clio
'muse' is the definition.
(Clio is an example)
'51 in company' is the wordplay.
'51' becomes 'li' (roman numeral).
'in' means one lot of letters goes inside another.
'company' becomes 'co' (abbreviation).
'li' placed inside 'co' is 'CLIO'.
'of' is the link.
(Other definitions for clio that I've seen before include "Muse of history - car" , "1 across's Muse - from Renault?" , "Muse of history (from Renault?)" , "One of the Muses" , "In Greek and Roman myth, the Muse of history" .)