Heavyweight initially training cricket side (3)
I believe the answer is:
ton
'heavyweight' is the definition.
(I've seen this before)
'initially training cricket side' is the wordplay.
'initially' suggests taking the first letters.
'cricket side' becomes 'on'.
The first letter of 'training' is 't'.
't'+'on'='TON'
(Other definitions for ton that I've seen before include "Unit of weight equivalent to 2240 pounds" , "See 25" , "Two thousand, two hundred and forty lbs avoirdupois" , "20cwt" , "A slang 100 mph" .)