Emergency crew's first on river, the Thames at Oxford (6)
I believe the answer is:
crisis
'emergency' is the definition.
(thesaurus)
'crew's first on river the thames at oxford' is the wordplay.
'first' says to take the initial letters.
'on' is a charade indicator (letters next to each other).
'river' becomes 'r'.
'the thames at oxford' becomes 'isis' (I've seen this before).
The initial letter of 'crews' is 'c'.
'c'+'r'+'isis'='CRISIS'
(Other definitions for crisis that I've seen before include "a difficult situation" , "Crucial and dangerous moment" , "Crucial or decisive moment" , "Moment of danger" , "Decisive moment, turning point" .)