Black underpants supporting several balls - that's too much of a load (10)
I believe the answer is:
overburden
'too much of a load' is the definition.
(an excessive burden)
'black underpants supporting several balls' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'underpants' becomes 'urden' (anagram of under - 'pants' is an anagram indicator).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'several balls' becomes 'over' (cricket).
'b'+'urden'='burden'
'burden' put after 'over' is 'OVERBURDEN'.
'that's' acts as a link.
(Other definitions for overburden that I've seen before include "Load excessively" , "demand too much work" , "Worry too much" .)