Running out of it can be tiresome (5)
I believe the answer is:
steam
'be tiresome' is the definition.
I can't tell whether this definition defines the answer.
'running out of it' is the wordplay.
'running' becomes 'seam' (I can't explain this - if you can you should give a lot more credence to this answer).
'out of' indicates putting letters inside (some letters on the outside of others).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'seam' enclosing 't' is 'STEAM'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Cook" , "Invisible gas from boiling water" , "Kind of power" , "Water vapour; impetus" , "Hot water suspended in air" .)