Not one younger, would you believe? (2,4)
I believe the answer is:
no less
'would you believe?' is the definition.
('no less' is said after a surprising claim)
'not one younger' is the wordplay.
'not' becomes 'no' (I've seen this in other clues).
'one younger' becomes 'less'.
'no'+'less'='NO LESS'
(Other definitions for no less that I've seen before include "Phrase used to emphasise admiration or surprise" , "Ironic comment - just as much" , "praise, ironically" , "would you believe!" , "as important" .)