Constant time is mine (3)
I believe the answer is:
pit
'is mine' is the definition.
(as in a mining pit)
'constant time' is the wordplay.
'constant' becomes 'pi' (mathematical constant).
'time' becomes 't' (abbreviation).
'pi'+'t'='PIT'
(Other definitions for pit that I've seen before include "Cherry stone" , "Take stone" , "Tip into hole" , "Quarry" , "Mine; fruit-stone" .)