Cockney believed that female ancestor (5)
I believe the answer is:
elder
'ancestor' is the definition.
(I've seen this before)
'cockney believed that female' is the wordplay.
'cockney believed' becomes ''eld' ('held' - 'hold' can be a synonym of 'believe' - said with the 'H' dropped - a cockney accent.).
'that female' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'eld'+'er'='ELDER'
Can you help me to learn more?
(Other definitions for elder that I've seen before include "one with roots" , "Earlier born" , "Official of some Churches, or senior statesman" , "Tribal advisor" , "Bush" .)