Picking up (9)
(Other definitions for answering that I've seen before include "Acknowledging" , "Replying" , "Picking up receiver" , "Responding" .)
'picking up' is the definition.
('sense' can be a synonym of 'pick up')
This is the entire clue.
(Other definitions for sensing that I've seen before include "Ensigns (anag)" , "Having a suspicion" .)