Assess too low, but aunt erred badly (9)
I believe the answer is:
underrate
'assess too low' is the definition.
I can't tell whether this definition defines the answer.
'aunt erred badly' is the wordplay.
'badly' is an anagram indicator.
'aunt'+'erred'='aunterred'
'aunterred' with letters rearranged gives 'UNDERRATE'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for underrate that I've seen before include "Endure art, think of too little value" , "Value insufficiently, think too little of" , "Not value enough" , "Think too little of, fail to appreciate" , "Have too low an opinion of" .)