Follow the track when he has departed (5)
I believe the answer is:
trail
'follow' is the definition.
(trail can mean to follow in secret)
'the track when he has departed' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'track' could be 'rail' (synonyms) and 'rail' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trail that I've seen before include "Fall behind" , "Drag; path" , "Dog - track" , "Pursue" , "Spoor - lag behind" .)