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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'training' becomes 'PE' (short for Physical Education).
'runs' becomes 'r' (cricket abbreviation).
'ref' going within '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" .)