Treated shark close to mainland (6)
I believe the answer is:
nursed
'treated' is the definition.
(nursing is a kind of treating)
'shark close to mainland' is the wordplay.
'shark' becomes 'nurse' (nurse shark is a kind of shark).
'close to' suggests the final letters (the close/ending of).
The final letter of 'mainland' is 'd'.
'nurse'+'d'='NURSED'
(Other definitions for nursed that I've seen before include "26dn the ill" , "Took care of the sick" , "Cherished" , "Carefully treated" , "Sunder (anag)" .)