Ground’s breadth unchanged (5)
I believe the answer is:
basis
'ground's' is the definition.
(synonyms)
'breadth unchanged' is the wordplay.
'breadth' becomes 'b' (mathematical abbreviation).
'unchanged' becomes 'as is' (as things currently are).
'b'+'asis'='BASIS'
(Other definitions for basis that I've seen before include "Underlying support (for an idea)" , "grounds" , "Reason" , "Fundamental part" , "Foundation, fundament" .)