Excited, I start to gambol between rugby posts (4)
I believe the answer is:
high
'excited' is the definition.
(I've seen this in other clues)
'i start to gambol between rugby posts' is the wordplay.
'start to' indicates taking the first letters.
'between' means one lot of letters goes inside another.
'rugby posts' becomes 'HH' (rugby goalposts resemble the letter H).
The initial letter of 'gambol' is 'g'.
'i'+'g'='ig'
'ig' placed inside 'hh' is 'HIGH'.
(Other definitions for high that I've seen before include "Tall - loud - under the influence of drugs" , "State of euphoria" , "Large, complex (and shady?) money transactions" , "Over-excited" , "Relatively great or specific elevation" .)