Insecure shifting dunes at end of bay (8)
I believe the answer is:
unsteady
'insecure' is the definition.
(thesaurus)
'shifting dunes at end of bay' is the wordplay.
'shifting' indicates an anagram.
'end of' indicates one should take the final letters.
The final letter of 'bay' is 'y'.
'dunes'+'at'='dunesat'
'dunesat' with letters rearranged gives 'unstead'.
'unstead'+'y'='UNSTEADY'
(Other definitions for unsteady that I've seen before include "not firmly fixed" , "Wobbly" , "likely to wobble" , "Not sure" , "Reeling" .)