Possibly please when slumbering (6)
I believe the answer is:
asleep
'slumbering' is the definition.
(similar in meaning)
'possibly please' is the wordplay.
'possibly' indicates anagramming the letters.
'please' with letters rearranged gives 'ASLEEP'.
'when' is the link.
(Other definitions for asleep that I've seen before include "In the arms fo Morpheus" , "in bed?" , "Knitting up the ravelled sleeve of care" , "(Of a limb) numb" , "Please (anag) - numb" .)