Interact regularly — that's efficient (4)
I believe the answer is:
neat
'efficient' is the definition.
(I've seen this before)
'interact regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'interact' are 'NEAT'.
'that's' acts as a link.
(Other definitions for neat that I've seen before include "bull, eg" , "without ice?" , "Smart" , "Ordered; without water" , "Steer - straight - trim" .)