In a trice, perhaps, the dunderhead gets a loan! (6)
I believe the answer is:
credit
'a loan' is the definition.
(thesaurus)
'in a trice perhaps the dunderhead' is the wordplay.
'in' is an insertion indicator.
'perhaps' is an anagram indicator.
'the dunderhead' becomes 'd' (head letter of 'dunder').
'trice' anagrammed gives 'creit'.
'creit' going around 'd' is 'CREDIT'.
'gets' acts as a link.
(Other definitions for credit that I've seen before include "Acknowledgement of a contributor" , "good character" , "Add (money) to account" , "Adam and Eve" , "Belief - trust - favourable acknowledgment" .)