Delayed training in troublesome communications app (5)
I believe the answer is:
skype
'communications app' is the definition.
(I know that Skype is a mobile app)
'delayed training in troublesome' is the wordplay.
I cannot really see how this works, but
'training' could be 'pe' (Physical Education) and 'pe' is found within the answer.
The remaining letters 'sky' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for skype that I've seen before include "Online phone service" , "Have a spoken conversation via the internet" , "Computer-based telephone software" , "Telephone over the internet" , "Way of calling" .)