Audible drop in level (4)
I believe the answer is:
tier
'level' is the definition.
(I know that level can be written as tier)
'audible drop' is the wordplay.
'audible' indicates a 'sounds like' (homophone) clue.
'drop' becomes 'tear' (teardrop).
'tear' sounds like 'TIER'.
'in' acts as a link.
(Other definitions for tier that I've seen before include "One making knot - level" , "one doing Bond" , "I might draw" , "Level within hierarchy" , "One securing" .)