Spots run when opening for Hampshire (4)
I believe the answer is:
rash
'spots' is the definition.
(I've seen this before)
'run when opening for hampshire' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'opening for' says to take the initial letters.
The first letter of 'hampshire' is 'h'.
'r'+'as'+'h'='RASH'
(Other definitions for rash that I've seen before include "Impetuous; skin ailment" , "Impulsive, having skin ailment" , "Lacking in caution" , "Red area on skin" , "Over-hasty, reckless" .)