Priest under extremely heavy layer makes short dash (6)
I believe the answer is:
hyphen
'short dash' is the definition.
'priest under extremely heavy layer' is the wordplay.
'priest' becomes 'p' (ecclesiastical abbreviation).
'under' means one lot of letters go next to another.
'extremely' means to remove the middle letters.
'layer' becomes 'hen' (I've seen this before).
'heavy' with its centre taken out is 'hy'.
'p' after 'hy' is 'hyp'.
'hyp'+'hen'='HYPHEN'
'makes' acts as a link.
(Other definitions for hyphen that I've seen before include "Word-joining mark" , "A horizontal word-joiner" , "Stroke between syllables or words" , "Mark between parts of a compound word" , "Line in a double-barrelled name, say" .)