Protagonist could be tied up when shown sequence of numbers and letters (15)

'protagonist could be tied up when shown sequence of numbers' is the definition.
I know nothing about this answer so I can't tell whether this works.

'and letters' is the wordplay.
I cannot really see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
'letters' could be 'characters' and 'characters' is present in the answer.
The remaining letters 'trig' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?