Beat the unbeatable (4)
I believe the answer is:
best
'unbeatable' is the definition.
(I've seen this before)
'beat the' is the wordplay.
'beat' becomes 'bes' (I can't explain this - if you can you should give a lot more credence to this answer).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'bes'+'t'='BEST'
Can you help me to learn more?
(Other definitions for best that I've seen before include "Most outstanding" , "Most excellent - late football star" , "former football star" , "erstwhile drummer" , "Most suitably" .)