Breach your old limits or be frugal (7)
I believe the answer is:
thrifty
'be frugal' is the definition.
(similar in meaning)
'breach your old limits' is the wordplay.
'breach' becomes 'rift' (I've seen this in another clue).
'your old' becomes 'thy' ('thy' historically meant 'your').
'limits' is an insertion indicator (limit can mean to contain or block off).
'rift' put within 'thy' is 'THRIFTY'.
'or' acts as a link.
(Other definitions for thrifty that I've seen before include "Careful with money" , "Conserving" , "Frugal - economical" , "Careful and diligent in the use of resources" , "Not extravagant" .)