Closest sea tern is disturbed (7)
I believe the answer is:
nearest
'closest' is the definition.
(synonyms)
'sea tern is disturbed' is the wordplay.
'is disturbed' indicates anagramming the letters (the order of letters is disturbed).
'sea'+'tern'='seatern'
'seatern' anagrammed gives 'NEAREST'.
(Other definitions for nearest that I've seen before include "Most adjacent" , "Next" , "Sneer at (anag)" , "Most intimate" , "Most accessible" .)