United with revolutionary Tamil Tigers' leader in occupation - hesitation creates threat (9)
I believe the answer is:
ultimatum
'threat' is the definition.
(I've seen this before)
'united with revolutionary tamil tigers' leader in' is the wordplay.
'united' becomes 'U' (as in Man U, English football team).
'with' says to put letters next to each other.
'revolutionary' is an anagram indicator.
'tigers' leader in' becomes 'tum' (I can't explain this - if you can you should believe this answer much more).
'tamil' is an anagram of 'ltima'.
'u'+'ltima'+'tum'='ULTIMATUM'
'occupation hesitation creates' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ultimatum that I've seen before include "Threat" , "Final point" , "Final demand or statement of terms" , "Final (peremptory) demand" , "Uncompromising demand" .)