Activity during sleep contributes to fair amount lost (9)
I believe the answer is:
decrement
'amount lost' is the definition.
I can't tell whether this defines the answer.
'activity during sleep contributes to fair' is the wordplay.
'activity during sleep' becomes 'REM' (rapid eye movement, a phase of sleep).
'contributes to' is an insertion indicator.
'fair' becomes 'decent'.
'rem' inserted within 'decent' is 'DECREMENT'.
Can you help me to learn more?
(Other definitions for decrement that I've seen before include "Lost amount" , "Reduction, diminution" .)