Jack enters sports day - it goes off with a bang (6)
I believe the answer is:
petard
'it goes off with a bang' is the definition.
The definition suggests a singular noun which matches the answer.
'jack enters sports day' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'enters' indicates putting letters inside.
'sports' becomes 'pe' (Physical Education).
'day' becomes 'd' (abbreviation for day).
'pe'+'d'='ped'
'tar' placed within 'ped' is 'PETARD'.
Can you help me to learn more?
(Other definitions for petard that I've seen before include "Hoist with one's own -" , "charged case" , "Something likely to explode" , "Small bomb; one to be hoist with" , "Explosive device, one could have been hoist by their own, of old" .)