‘I’ll Be with You in a Tick’ Means ‘I’ll Be with You Very Soon’

‘I'll be with you in a tick’ is natural when a shop assistant is occupied or a colleague needs to finish one small task first. It means ‘I'll attend to you very soon’, not after one literal tick of a clock. Here, a tick is an informal, very short period, and the phrase gently asks the listener to wait. For example: ‘Can you check this?’ ‘In a tick—I'm just finishing this email.’ It is familiar in Australian and British English; American speakers are more likely to say in a sec or in a minute.

https://dictionary.cambridge.org/dictionary/english/tick