Thick-headed pair, furthermore (3)
I believe the answer is:
too
'furthermore' is the definition.
(I've seen this before)
'thick-headed pair' is the wordplay.
'thick-headed' becomes 't' (this could be a standard abbreviation which I don't know about).
'pair' becomes 'oo' (resembles 00, a pair of ducks in cricket).
't'+'oo'='TOO'
Can you help me to learn more?
(Other definitions for too that I've seen before include "Also; overly" , "Additionally" , "What a pity!" , "As well; in addition" , "On top - also" .)