They are timid yet pipe up (4)
I believe the answer is:
deer
'they are timid' is the definition.
The definition suggests an adverb but the answer is not.
'pipe up' is the wordplay.
'pipe' becomes 'reed pipe' (reed pipe is a kind of pipe).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'reed' written backwards gives 'DEER'.
'yet' acts as a link.
Can you help me to learn more?
(Other definitions for deer that I've seen before include "Like a power switch" , "maybe does" , "Antler-bearing animal" , "Antlered mammal" , "Perhaps fallow" .)