I’m surprised garden barrier’s lost a little height (3)
I believe the answer is:
aha
'i'm surprised' is the definition.
(expression of surprise)
'garden barrier's lost a little height' is the wordplay.
'garden barrier' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'lost' indicates an anagram.
'little' says to take the initial letters.
The first letter of 'height' is 'h'.
'a'+'h'='ah'
'ah' anagrammed gives 'ha'.
'a'+'ha'='AHA'
Can you help me to learn more?
(Other definitions for aha that I've seen before include "I see!" , "I've got it!" , "Exclamation of triumph or mockery, a palindrome" , "Expression of surprise, satisfaction or triumph" , "it expresses surprise" .)