Harassed policemen losing months of time when mammals appeared (8)
I believe the answer is:
pliocene
'time when mammals appeared' is the definition.
(geological epoch)
'harassed policemen losing months' is the wordplay.
'harassed' indicates anagramming the letters.
'losing' is a deletion indicator.
'months' becomes 'm'.
'policemen' with 'm' removed is 'policeen'.
'policeen' with letters rearranged gives 'PLIOCENE'.
'of' acts as a link.
(Other definitions for pliocene that I've seen before include "from years ago" , "Early epoch" , "Epoch when first hominids arose; lone epic (anag.)" , "Period" .)