Period where one might find training difficult? (8)

'one might find training difficult?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'period' is the wordplay.
I cannot quite understand how this works, but
'period' could be 'top' (top is a kind of period) and 'top' is found in the answer.
The remaining letters 'fulls' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'where' is the link.

Can you help me to learn more?