Science that puts healthy colour in a lad (6)
I believe the answer is:
botany
'science' is the definition.
(botany is a kind of science)
'healthy colour in a lad' is the wordplay.
'healthy colour' becomes 'tan'.
'in' is an insertion indicator.
'a lad' becomes 'boy' (lad means a young boy).
'tan' inserted into 'boy' is 'BOTANY'.
'that puts' is the link.
(Other definitions for botany that I've seen before include "Unlucky, doomed" , "the name of a bay" , "The scientific study of plants" , "plants growing in one area" , "Plant study" .)