Negotiable unit held by me for some time (6)
I believe the answer is:
minute
'some time' is the definition.
(unit of time)
'negotiable unit held by me' is the wordplay.
'negotiable' is an anagram indicator.
'held by' is an insertion indicator.
'unit' is an anagram of 'inut'.
'inut' inserted inside 'me' is 'MINUTE'.
'for' acts as a link.
(Other definitions for minute that I've seen before include "Formal note" , "Barely perceptible" , "official memorandum" , "Extremely small" , "60 seconds; tiny" .)