Create bridge to carry weight (5)
I believe the answer is:
spawn
'create' is the definition.
(I know that spawn is a type of cause)
'bridge to carry weight' is the wordplay.
'bridge' becomes 'span' (synonyms).
'to carry' means one lot of letters goes inside another.
'weight' becomes 'w' (short for weight).
'span' placed around 'w' is 'SPAWN'.
(Other definitions for spawn that I've seen before include "Eggs of fish, frogs, etc" , "Give rise to; fish egg" , "Frog eggs" , "Generate; fish eggs" , "Mass of eggs laid in water" .)