This beast ultimately tried to attract partner and swaggered (8)
I believe the answer is:
strutted
'swaggered' is the definition.
('strut' can be a synonym of 'swagger')
'this beast ultimately tried to attract partner' is the wordplay.
'ultimately' says to take the final letters.
'tried to attract partner' becomes 'rutted' (animals rut in mating season).
The final letters of 'this beast' are 'st'.
'st'+'rutted'='STRUTTED'
'and' is the link.
(Other definitions for strutted that I've seen before include "Supported" , "showed off" , "Having support" , "Swaggered" , "Your stuff may be" .)