In quakes, most err badly (7)
I believe the answer is:
tremors
'in quakes' is the definition.
'tremors' can be an answer for 'quakes' ('tremor' can be a synonym of 'quake'). I'm unsure of the 'in' bit.
'most err badly' is the wordplay.
'badly' indicates an anagram.
'most'+'err'='mosterr'
'mosterr' is an anagram of 'TREMORS'.
Can you help me to learn more?
(Other definitions for tremors that I've seen before include "Small earthquakes" , "Err most (anag.)" , "Vibrations" , "Quivering movements" , "Quakings" .)