Rather worried as tide covers middle of dunes (7)
I believe the answer is:
instead
'rather' is the definition.
('instead' can be a synonym of 'rather')
'worried as tide covers middle of dunes' is the wordplay.
'worried' indicates an anagram.
'covers' is an insertion indicator.
'middle of' means to look at the middle letters.
The middle of 'dunes' is 'n'.
'as'+'tide'='astide'
'astide' anagrammed gives 'istead'.
'istead' placed around 'n' is 'INSTEAD'.
(Other definitions for instead that I've seen before include "As a substitute" , "Stained (anag)" , "use this as a replacement" , "Rather - detains (anag)" , "Alternatively" .)