Demonstrate removing mass for bridging structure (4)
I believe the answer is:
arch
'structure' is the definition.
(arch is a kind of structure)
'demonstrate removing mass' is the wordplay.
'demonstrate' becomes 'march' (synonyms).
'removing' is a deletion indicator.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'march' with 'm' taken out is 'ARCH'.
'for bridging' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for arch that I've seen before include "Roguish, teasing" , "Bend into upward curve" , "Structure with a keystone" , "Wellington's, perhaps" , "Char the curved structure" .)