Speaking about whistle part (3)
I believe the answer is:
pea
'whistle part' is the definition.
(I've seen this in another clue)
'speaking about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'PEA' can be found hidden inside 'speaking'.
(Other definitions for pea that I've seen before include "Pulse vegetable" , "Climber" , "Common vegetable seed from pod" , "veg" , "Climbing plant; vegetable" .)