Bishop collapsed under load (6)
I believe the answer is:
burden
'load' is the definition.
(I know that load can be written as burden)
'bishop collapsed under' is the wordplay.
'bishop' becomes 'b' (chess abbreviation).
'collapsed' indicates anagramming the letters.
'under' is an anagram of 'urden'.
'b'+'urden'='BURDEN'
(Other definitions for burden that I've seen before include "Weigh down - load" , "Oppressive duty" , "chorus" , "Encumbrance" , "Main theme (of book)" .)