Misjudge interest due — am struggling (13)
I believe the answer is:
underestimate
'misjudge' is the definition.
(underestimating is a kind of misjudging)
'interest due am struggling' is the wordplay.
'struggling' indicates anagramming the letters (the letters struggle or wriggle into a new form).
'interest'+'due'+'am'='interestdueam'
'interestdueam' is an anagram of 'UNDERESTIMATE'.
(Other definitions for underestimate that I've seen before include "Sell short" , "Regard (someone) as less capable than is the case" , "inaccurate" , "Rate as too low" , "Evaluation that is too low" .)