They're of wood, part of it in bulk (5)
I believe the answer is:
masts
'they're of wood part' is the definition.
The definition suggests an adverb but the answer is not.
'it in bulk' is the wordplay.
'it' becomes 't' (abbreviation. e.g. in 'tis).
'in' means one lot of letters goes inside another.
'bulk' becomes 'mass' (synonyms).
't' going within 'mass' is 'MASTS'.
'of' is the link.
Can you help me to learn more?
(Other definitions for masts that I've seen before include "Eg, transmitter towers" , "things for flags" .)