'Fight Club' title, losing it (6)
I believe the answer is:
battle
'fight' is the definition.
(battling is a kind of fighting)
'club title losing it' is the wordplay.
'club' becomes 'bat' (I've seen this before**).
'losing' is a deletion indicator.
'title' with 'it' removed is 'tle'.
'bat'+'tle'='BATTLE'
(Other definitions for battle that I've seen before include "Sussex town" , "Major engagement" , "Midway" , "- - of 5 down" , "Eg, Bosworth, Blenheim" .)