OK, a quiet one is the forest giraffe (5)
I believe the answer is:
okapi
'the forest giraffe' is the definition.
'okapi' can be an answer for 'giraffe' (okapi is a kind of giraffe). I'm not certain of the 'the forest' bit.
'ok a quiet one' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'one' becomes 'i' (Roman numeral).
'ok'+'a'+'p'+'i'='OKAPI'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for okapi that I've seen before include "African leaves fancier" , "Horned ruminant" , "African mammal related to giraffe" , "Animal of the giraffe family (much loved by crossword setters)" , "Central African animal" .)