Fell over during shortened rest (5,3)
I believe the answer is:
chill out
'rest' is the definition.
(I've seen this before)
'fell over during shortened' is the wordplay.
I cannot really understand how this works, but
'fell' could be 'hill' (I've seen this before) and 'hill' is found within the answer.
'over' could be 'o' (cricket abbreviation) and 'o' is found within the answer.
'shortened' could be 'cut' (synonyms) and 'cut' is found within the leftover letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for chill out that I've seen before include "Take it easy" , "Relax after exercise" , "Relax (informal)" , "Relax (inf.)" , "Relax after energetic spell" .)