50 in club to complain (5)
I believe the answer is:
bleat
'complain' is the definition.
(I know this)
'50 in club' is the wordplay.
'50' becomes 'l' (Roman numeral).
'in' means one lot of letters goes inside another.
'club' becomes 'beat' (I've seen this before).
'l' inserted inside 'beat' is 'BLEAT'.
'to' is the link.
(Other definitions for bleat that I've seen before include "Cry like a goat" , "Make a noise like a sheep" , "Cry plaintively" , "Lamb's cry" , "Cry of animal" .)