Keep putting in at a different rate (6)
I believe the answer is:
retain
'keep' is the definition.
(I know that keep is a more specific form of the action keep up)
'in at a different rate' is the wordplay.
'at' says to put letters next to each other (I've seen this in other clues).
'a different' indicates an anagram (letters placed in a different order).
'rate' anagrammed gives 'reta'.
'in' put after 'reta' is 'RETAIN'.
'putting' acts as a link.
(Other definitions for retain that I've seen before include "Hire - keep" , "'Hold, keep back (6)'" , "Hold onto" , "Keep in place" , "Continue to hold on to" .)