Overburdened with word (4)
I believe the answer is:
verb
'word' is the definition.
(I know that verb is a type of content word)
'overburdened with' is the wordplay.
'with' says the answer is hidden in the clue.
'VERB' is hidden in the letters of 'overburdened'.
(Other definitions for verb that I've seen before include "Word such as run, make, or do" , "love, for example" , "'In grammar, it denotes action (4)'" , "Maybe protest" , "Part of speech" .)