Decorated old Royal Navy mate losing millions (6)
I believe the answer is:
ornate
'decorated' is the definition.
(similar in meaning)
'old royal navy mate losing millions' is the wordplay.
'old' becomes 'o' (abbreviation).
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'losing' is a deletion indicator.
'millions' becomes 'm' (abbreviation).
'mate' with 'm' removed is 'ate'.
'o'+'rn'+'ate'='ORNATE'
(Other definitions for ornate that I've seen before include "Highly decorated" , "Richly decorative" , "Florid" , "Detailed" , "Over-elaborate" .)