Upset stomach - onset of headache's gone - that's lucky (6)
I believe the answer is:
mascot
'lucky' is the definition.
The answer and definition are not the same part of speech. However, I know that 'lucky' relates to this answer.
'upset stomach onset of headache's gone that's' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found within the answer.
'that's' could be 'sc' (short for Latin scilicet- 'it follows' or 'that is) and 'sc' is located in the answer.
The remaining letters 'mat' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mascot that I've seen before include "Symbolic figure, of a team perhaps" , "Lucky person" , "It brings good luck" , "Symbolic lucky figure" , ".Person or thing believed to bring good luck" .)