Reservation about navy being toasted (5)
I believe the answer is:
burnt
'being toasted' is the definition.
(toast can mean to heat up or burn)
'reservation about navy' is the wordplay.
'reservation' becomes 'but' (a 'but' is a reservation or objection to something).
'about' is an insertion indicator.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'but' placed around 'rn' is 'BURNT'.
(Other definitions for burnt that I've seen before include "Ruined by overcooking" , "affected by singers?" , "Incinerated" , "Charred" , "Consumed by fire" .)