Each year end increase is an obstacle for auditors (6)
I believe the answer is:
earwax
'an obstacle for auditors' is the definition.
(can block ears)
'each year end increase' is the wordplay.
'each' becomes 'ea' (abbreviation).
'end' says to take the final letters.
'increase' becomes 'wax' (waxing is a kind of increasing).
The final letter of 'year' is 'r'.
'ea'+'r'+'wax'='EARWAX'
'is' acts as a link.
(Other definitions for earwax that I've seen before include "Substance accumulating in the lugholes" , "Soft yellow secretion - cerumen" , "Secretion from aural glands" , "Otic deposit" , "Soft yellow substance secreted by glands of a certain organ" .)