23 across
Greatly
affect
stupendous
hosts
(5)
Greatly affect stupendous hosts (5)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
The definition is Greatly affect — to upend something is to overturn or disrupt it. The sneaky bit is that the answer is hiding inside stupendous (st-UPEND-ous), with hosts tipping you off that the word you want is tucked inside another one.