A tad upset, chap goes in hard! (7)
I believe the answer is:
adamant
'hard' is the definition.
(I've seen this before)
'a tad upset chap goes in' is the wordplay.
'upset' says the letters should be written in reverse.
'chap' becomes 'Man' (chap can mean a man).
'goes in' indicates putting letters inside.
'tad' reversed gives 'dat'.
'dat' going around 'man' is 'damant'.
'a'+'damant'='ADAMANT'
(Other definitions for adamant that I've seen before include "Fabulous rock" , "Hard stone" , "Completely determined" , "Impervious to pleas or persuasion" , "unshakeable in purpose" .)