Tailor felt alarmed clothing one person in navy (5,7)

'person in navy' is the definition.
(rank in the navy)

'tailor felt alarmed clothing one' is the wordplay.
'tailor' indicates anagramming the letters (I've seen this in other clues).
'clothing' indicates putting letters inside (some letters cover up or clothe others).
'one' becomes 'i' (Roman numeral).
'felt'+'alarmed'='feltalarmed'
'feltalarmed' with letters rearranged gives 'fleetadmral'.
'fleetadmral' placed around 'i' is 'FLEET ADMIRAL'.