Negotiable unit held by me for some time (6)
I believe the answer is:
minute
'some time' is the definition.
(I've seen this before)
'negotiable unit held by me' is the wordplay.
'negotiable' indicates an anagram.
'held by' means one lot of letters goes inside another.
'unit' is an anagram of 'inut'.
'inut' going into '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" .)