Taxi picked up monarch's funds (7)
I believe the answer is:
backing
'funds' is the definition.
(thesaurus)
'taxi picked up monarch's' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'picked up' shows that the letters should be reversed in order.
'monarch' becomes 'king' (king is a kind of monarch).
'cab' in reverse letter order is 'bac'.
'bac'+'king'='BACKING'
(Other definitions for backing that I've seen before include "funds?" , "Withdrawing" , "Endorsement" , "gambling" , "Support" .)