Pleased to be Gordonstoun's head boy (4)
I believe the answer is:
glad
'pleased' is the definition.
(similar in meaning)
'gordonstoun's head boy' is the wordplay.
'head' suggests taking the first letters.
'boy' becomes 'lad' (informal term for boy).
The first letter of 'gordonstoun' is 'g'.
'g'+'lad'='GLAD'
'to be' acts as a link.
(Other definitions for glad that I've seen before include "bright?" , "Elated" , "happiness" , "Feeling pleasure" , "Dressy clothes, informally .... rags" .)