Like a nobleman’s double, taking in Commons at first (5)
I believe the answer is:
ducal
'like a nobleman's' is the definition.
(I've seen this before)
'double taking in commons at first' is the wordplay.
'double' becomes 'dual' (synonyms).
'taking in' means one lot of letters goes inside another.
'at first' indicates taking the first letters.
The initial letter of 'commons' is 'c'.
'dual' going around 'c' is 'DUCAL'.
(Other definitions for ducal that I've seen before include "Relating to a nobleman" , "Relating to a duke" , "Peer's" , "Relating to the highest British hereditary title" , "Representing noble" .)