Heartbroken to block dubious trade in pyramids (10)
I believe the answer is:
tetrahedra
'pyramids' is the definition.
(a tetrahedron is a triangular pyramid)
'heartbroken to block dubious trade' is the wordplay.
'heartbroken' becomes 'etrah' (anagram of 'heart').
'to block' is an insertion indicator (upper walls off glass inside formal).
'dubious' indicates anagramming the letters.
'trade' is an anagram of 'tedra'.
'etrah' placed within 'tedra' is 'TETRAHEDRA'.
'in' is the link.
(Other definitions for tetrahedra that I've seen before include "Figures" , "Triangular pyramids" , "These are solid" .)