Stickler for niceties breaking down in physical training (13)
I believe the answer is:
perfectionist
'stickler' is the definition.
(I have seen 'The Stickler' mean 'perfectionist' so perhaps 'stickler' could also mean 'perfectionist')
'for niceties breaking down in physical training' is the wordplay.
'breaking down' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'physical training' becomes 'pt'.
'for'+'niceties'='forniceties'
'forniceties' is an anagram of 'erfectionis'.
'erfectionis' inserted inside 'pt' is 'PERFECTIONIST'.
(Other definitions for perfectionist that I've seen before include "The Stickler" , "One aiming for the best" , "One seeking the ideal" , "Striver for the ideal" , "I expect the best" .)