Finally loses cup being complacent (4)
I believe the answer is:
smug
'complacent' is the definition.
(smug can mean complacent or self-satisfied)
'finally loses cup' is the wordplay.
'finally' says to take the final letters.
'cup' becomes 'mug' (I've seen this before).
The last letter of 'loses' is 's'.
's'+'mug'='SMUG'
'being' acts as a link.
(Other definitions for smug that I've seen before include "Self-satisfied and vain" , "looking pleased with itself" , "Self-satisfied, complacent" , "Prim" , "Irritatingly pleased with oneself" .)