Worry about a single occasion with the navy (7)
I believe the answer is:
concern
'worry' is the definition.
(I know that worry can be written as concern)
'about a single occasion with the navy' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'a single occasion' becomes 'once' (I've seen this in another clue).
'with' says to put letters next to each other.
'the navy' becomes 'RN' (abbreviation for Royal Navy).
'c'+'once'+'rn'='CONCERN'
(Other definitions for concern that I've seen before include "Firm - interest" , "Relate to; anxiety" , "Involve" , "Disquiet" , "Business - worry" .)