I went off key in it and that’s a pain! (6)
I believe the answer is:
twinge
'that's a pain' is the definition.
(twinge is a kind of pain)
'i went off key in it' is the wordplay.
'off' indicates anagramming the letters.
'key' becomes 'g' (musical key).
'in it' means one lot of letters goes inside another.
'i'+'went'='iwent'
'iwent' with letters rearranged gives 'twine'.
'twine' going around 'g' is 'TWINGE'.
'and' is the link.
(Other definitions for twinge that I've seen before include "Sudden sharp feeling" , "Slight localised pain" , "Small pain eg from conscience" , "Qualm" , "Brief sharp pain" .)