Throughout Whitsun, a minister gets big wave (7)
I believe the answer is:
tsunami
'big wave' is the definition.
(a tsunami is a large wave)
'throughout whitsun a minister gets' is the wordplay.
'gets' indicates a hidden word.
'TSUNAMI' is hidden in the letters of 'throughout whitsun a minister'.
(Other definitions for tsunami that I've seen before include "Large ocean wave" , "Marine natural disaster" , "wall of water" , "Strong surge" , "stormy sea" .)