Control farm animal upset about start of rainstorm (4)
I believe the answer is:
grip
'control' is the definition.
(grip is a kind of control)
'farm animal upset about start of rainstorm' is the wordplay.
'farm animal' becomes 'pig' (I've seen this before**).
'upset' shows that the letters should be reversed in order.
'about' means one lot of letters goes inside another.
'start of' suggests taking the first letters.
The initial letter of 'rainstorm' is 'r'.
'pig' backwards is 'gip'.
'gip' enclosing 'r' is 'GRIP'.
(Other definitions for grip that I've seen before include "Member of film crew" , "Grab" , "Clutch - holdall" , "Grasp, clutch" , "Engage" .)