Isotonic solution chap brought into function (6)
I believe the answer is:
saline
'isotonic solution' is the definition.
(saline is a kind of isotonic solution)
'chap brought into function' is the wordplay.
'chap' becomes 'al' (**).
'brought into' is an insertion indicator.
'function' becomes 'sine' (mathematical function).
'al' placed within 'sine' is 'SALINE'.
(Other definitions for saline that I've seen before include "Containing or tasting of salt" , "like the Mediterranean?" , "Salty to aliens" , "solution" , "Containing salt or salts" .)