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' means one lot of letters goes inside another.
'unit' with letters rearranged gives 'inut'.
'inut' placed 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" .)