End of rum — crew ultimately upset (7)
I believe the answer is:
oddment
'end' is the definition.
(synonyms)
'rum crew ultimately upset' is the wordplay.
'rum' becomes 'odd' (rum can mean strange or dubious).
'crew' becomes 'men' (I've seen this before).
'ultimately upset' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'odd'+'men'+'t'='ODDMENT'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for oddment that I've seen before include "One of a broken set" , "Scrap" , "a bit spare" , "Just a bit left" , "Curiosity, peculiarity" .)