Shetland island qualified as rocky (8)
I believe the answer is:
unstable
'rocky' is the definition.
(similar in meaning)
'shetland island qualified' is the wordplay.
'shetland island' becomes 'unst' (an isle of the Shetland Islands).
'qualified' becomes 'able' (I've seen this before).
'unst'+'able'='UNSTABLE'
'as' is the link.
(Other definitions for unstable that I've seen before include "Likely to collapse" , "Shaky" , "Not well-balanced, liable to wobble" , "lacking security" , "Prone to mood swings" .)