Choose person who'll blow whistle in training runs (6)
I believe the answer is:
prefer
'choose person who'll' is the definition.
'prefer' can be an answer for 'choose' (synonyms). I'm not certain of the 'person who'll' bit.
'blow whistle in training runs' is the wordplay.
'blow whistle' becomes 'ref' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'training' becomes 'PE' (short for Physical Education).
'runs' becomes 'r' (cricket abbreviation).
'ref' put into 'pe' is 'prefe'.
'prefe'+'r'='PREFER'
Can you help me to learn more?
(Other definitions for prefer that I've seen before include "as one may with charges" , "support" , "Choose as more desirable, like better" , "Promote; like better" , "Like (something) better than another" .)