Sentimentality of last character abandoning Mediterranean island (3)
I believe the answer is:
goo
'sentimentality' is the definition.
(informal term for sentimentality)
'last character abandoning mediterranean island' is the wordplay.
'last character' becomes 'z'.
'abandoning' is a deletion indicator.
'mediterranean island' becomes 'gozo' (I've seen this before).
'gozo' with 'z' removed is 'GOO'.
'of' acts as a link.
(Other definitions for goo that I've seen before include "Sticky and slimy stuff" , "Substance that's viscous" , "Sticky substance (colloq.)" , "Sticky substance (sl.)" , "mud, for example" .)