At first extreme weight held up fame (4)
I believe the answer is:
note
'fame' is the definition.
(thesaurus)
'at first extreme weight held up' is the wordplay.
'at' means one lot of letters go next to another.
'first extreme' becomes 'e' (1st letter of 'extreme').
'weight' becomes 'ton' (ton is a kind of weight).
'held up' is a reversal indicator.
'e' put next to 'ton' is 'eton'.
'eton' reversed gives 'NOTE'.
(Other definitions for note that I've seen before include "Memo; pay attention to" , "Short message; piano key" , "Tone of a short record" , "Pay close attention to" , "Detect and remember" .)