Complaint letters periodically dispatched from island (3)
I believe the answer is:
sad
'complaint' is the definition.
(seasonal affective disorder, a medical complaint)
'letters periodically dispatched from island' is the wordplay.
'letters periodically dispatched from' indicates alternate letters (letters removed periodically, at regular intervals).
The alternate letters of 'island' are 'SAD'.
(Other definitions for sad that I've seen before include "upset" , "Despondent" , "grey" , "low" , "Melancholy, triste" .)