Give gear mistakenly, causing distress (8)
I believe the answer is:
aggrieve
'distress' is the definition.
(thesaurus)
'give gear mistakenly' is the wordplay.
'mistakenly' indicates anagramming the letters.
'give'+'gear'='givegear'
'givegear' anagrammed gives 'AGGRIEVE'.
'causing' is the link.
(Other definitions for aggrieve that I've seen before include "Do an injustice to" , "upset" , "Ill-use" , "Cause to feel sorrow" , "Break the heart" .)