Crucial US city to important figure, on reflection (7)
I believe the answer is:
pivotal
'crucial' is the definition.
(similar in meaning)
'us city to important figure on reflection' is the wordplay.
'us city' becomes 'LA' (Los Angeles).
'important figure' becomes 'VIP' (very important person).
'on reflection' says the letters should be written in reverse.
'la'+'to'+'vip'='latovip'
'latovip' reversed gives 'PIVOTAL'.
(Other definitions for pivotal that I've seen before include "Of crucial importance" , "Crucial, critical" , "Central - decisive" , "Crucially important" , "Key" .)