Simple limits to mortgage rate (4)
I believe the answer is:
mere
'simple' is the definition.
(synonyms)
'limits to mortgage rate' is the wordplay.
'limits to' means to remove the middle letters (only the edge letters are used).
'rate' becomes 're' (I am not sure about this - if you are sure you should believe this answer much more).
'mortgage' with its middle removed is 'me'.
'me'+'re'='MERE'
Can you help me to learn more?
(Other definitions for mere that I've seen before include "Nothing more than is specified - or a poetic lake" , "Pool -- pure and simple" , "Insignificant" , "Unimpressive" , "Lake - French mother" .)