It's worn by one before the start of training (7)
I believe the answer is:
singlet
'start of training' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'it's worn by one before' is the wordplay.
I cannot really see how this works, but
'one' could be 'single' (similar in meaning) and 'single' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for singlet that I've seen before include "top gear" , "underclothing" , "One unaccompanied" , "Underwear" , "Sleeveless shirt" .)