One’s on the phone: appear to block ear (3)
I believe the answer is:
app
'one's on the phone' is the definition.
I cannot understand how the remainder of the clue works.
(Other definitions for app that I've seen before include "Software item" , "Software program for a mobile device" , "bit of software" , "Software for mobile devices, in short" , "Program for a smart phone, in short" .)