Top each off - that's a pain (8)
I believe the answer is:
headache
'that's a pain' is the definition.
(headache is a kind of pain)
'top each off' is the wordplay.
'top' becomes 'head' (head is a kind of top).
'off' indicates an anagram.
'each' anagrammed gives 'ache'.
'head'+'ache'='HEADACHE'
(Other definitions for headache that I've seen before include "nasty problem" , "Worry, trouble (colloq.)" , "Migraine" , "Severe problem" , "Cause of worry/trouble" .)