Support freshwater fish out of river (4)
I believe the answer is:
beam
'support' is the definition.
(I know that beam is a type of structural member)
'freshwater fish out of river' is the wordplay.
'freshwater fish' becomes 'bream' (bream is a kind of freshwater fish).
'out of' indicates named letters should be taken away.
'river' becomes 'r' (abbreviation for river).
'bream' with 'r' taken out is 'BEAM'.
(Other definitions for beam that I've seen before include "Smile; piece of timber" , "Smile, transmit (radio signal)" , "Shine" , "Ray of light - structural piece of wood" , "Give a wide smile" .)