Hear they'll blow top and become agitated if this is sewn on to hat (7)
I believe the answer is:
earflap
This could be a double definition.
'hear they'll blow top and become agitated if this is sewn on' is the first definition.
The definition suggests a singular noun which matches the answer.
'hat' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'to' acts as a link.
Can you help me to learn more?
(Other definitions for earflap that I've seen before include "Part of headgear" , "Winter hat feature" , "ski gear?" .)