Fish press gives added firmness (5,4)
I believe the answer is:
angle iron
'added firmness' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'fish press' is the wordplay.
'fish' becomes 'angle' (angle can mean to fish).
'press' becomes 'iron' (ironing is a kind of pressing).
'angle'+'iron'='ANGLE IRON'
'gives' is the link.
Can you help me to learn more?
(Other definitions for angle iron that I've seen before include "constructive support" , "L-shaped structural component" , "Strong supporter" , "structure might need this device" , "L-shaped support bar" .)