Carla gutted to leave Mediterranean island, but no big deal (5)
I believe the answer is:
minor
'no big deal' is the definition.
(both can mean unimportant)
'carla gutted to leave mediterranean island but' is the wordplay.
'gutted' indicates the central letters (I've seen 'guts' mean this (the insides of the word)).
'to' means one lot of letters go next to another (I've seen this in other clues).
'leave' suggests taking the first letters (I've seen 'left of' mean this (the leftmost letter in the word)).
'but' becomes 'no' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The centre of 'carla' is 'r'.
The initial letters of 'mediterranean island' is 'mi'.
'mi'+'no'='mino'
'r' after 'mino' is 'MINOR'.
(Other definitions for minor that I've seen before include "Junior" , "One hasn't reached legal majority" , "Like clubs" , "Inconsequential" , "Adolescent" .)