Jar opening, finger finally poked in (5)
I believe the answer is:
grate
'jar' is the definition.
(both can mean to disturb or annoy)
'opening finger finally poked in' is the wordplay.
'finally' suggests the final letters.
'poked in' becomes 'ate' (I can't justify this - if you can you should give a lot more credence to this answer).
The last letters of 'opening finger' are 'gr'.
'gr'+'ate'='GRATE'
Can you help me to learn more?
(Other definitions for grate that I've seen before include "Shred - jar" , "part of fireplace" , "in which Santa lands?" , "Great (anag.)" , "Rasp; fire holder" .)