Having brought with you in case, avail yourself of (6)
I believe the answer is:
unpack
'having brought' is the definition.
I can't tell whether this definition defines the answer.
'you in case avail yourself of' is the wordplay.
I cannot really see how this works, but
'you' could be 'u' (commonly used in online communication) and 'u' is present in the answer.
'case' could be 'pack' (packing is a kind of casing) and 'pack' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'with' acts as a link.
Can you help me to learn more?
(Other definitions for unpack that I've seen before include "Empty (a suitcase)" , "Remove from the wrapping" , "Remove items from a suitcase" , "Take (something from a suitcase)" , "Remove stuff from cases" .)