Appropriate American insurers withdraw (5)
I believe the answer is:
usurp
'appropriate' is the definition.
(thesaurus)
'american insurers withdraw' is the wordplay.
'american' becomes 'us'.
'insurers' becomes 'pru' (short for Prudential Assurance Company, UK firm).
'withdraw' says the letters should be written in reverse.
'pru' reversed gives 'urp'.
'us'+'urp'='USURP'
(Other definitions for usurp that I've seen before include "Assume by force" , "Take possession of, illegally" , "Expropriate" , "Take control illegally" , "Seize unlawfully" .)