Boxer starts to train young lad (5)
I believe the answer is:
tyson
'boxer' is the definition.
(Tyson Fury the boxer)
'starts to train young lad' is the wordplay.
'starts to' indicates taking the first letters.
'lad' becomes 'son' (lad can informally mean a young boy).
The first letters of 'train young' is 'ty'.
'ty'+'son'='TYSON'
(Other definitions for tyson that I've seen before include "American world heavyweight champion, b. 1966" , "US world heavyweight boxing champion, b.1966 -- English fast bowler, d. 2015" , "Undisputed world heavyweight champion, 1987" , "Mike ..., former US prizefighter" , "26 4 [RING MASTER]" .)