Pay for extra work during an unspecified period (8)
I believe the answer is:
overtime
'pay for extra work' is the definition.
(I've seen this before)
'during an unspecified period' is the wordplay.
'during' becomes 'over' (I've seen this in other clues).
'an unspecified period' becomes 'time' (synonyms. I am not sure about the 'unspecified' bit.).
'over'+'time'='OVERTIME'
Can you help me to learn more?
(Other definitions for overtime that I've seen before include "Work outside normal hours" , "Extra hours worked" , "Work beyond normal hours? Remove it" , "Hours worked and paid for beyond the usual amount" , "Extra hours (at work)" .)