It ignites quarrel and Grandad's in bits (5,4)
I believe the answer is:
spark plug
'it ignites' is the definition.
'quarrel and grandad's in bits' is the wordplay.
I cannot quite see how this works, but
'quarrel' could be 'spar' (I've seen this before) and 'spar' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for spark plug that I've seen before include "Ignition device in an internal-combustion engine" , "Device screwed into a cylinder head" , "starter" , "engine component" , "Electrical device fitted in the cylinder heads of an internal-combustion engine" .)