Jack thanks a raw beginner (3)
I believe the answer is:
tar
'jack' is the definition.
(both are informal terms for sailor)
'thanks a raw beginner' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'beginner' indicates taking the first letters.
The first letter of 'raw' is 'r'.
'ta'+'r'='TAR'
(Other definitions for tar that I've seen before include "Informal sailor or road surfacing material" , "Product of burning tobacco" , "Kind of rat for sailor" , "Product of smoking cigarettes" , "Road-making substance" .)