Fast to save island nurse, showing mercy (7)
I believe the answer is:
lenient
'showing mercy' is the definition.
(I've seen this in another clue)
'fast to save island nurse' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'to save' means one lot of letters goes inside another.
'island' becomes 'i' (geographical abbreviation).
'nurse' becomes 'EN' (abbreviation for enrolled nurse).
'i'+'en'='ien'
'lent' enclosing 'ien' is 'LENIENT'.
(Other definitions for lenient that I've seen before include "Not severe, merciful" , "Compassionate" , "Forgiving, soft" , "Inclined to let people off lightly" , "Merciful or easygoing" .)