Slackens off when some fleas escape (5)
I believe the answer is:
eases
'slackens off' is the definition.
(I've seen this in another clue)
'some fleas escape' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'EASES' is hidden in the letters of 'fleas escape'.
'when' is the link.
(Other definitions for eases that I've seen before include "Assuages or slackens" , "Facilitates" , "Relaxes" , "Calms" , "Softens" .)