Security measure fixed, containing iron, beside river (6,3)
I believe the answer is:
safety net
'security measure' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'fixed containing iron beside river' is the wordplay.
'fixed' becomes 'sat' (both can mean positioned).
'containing' indicates putting letters inside.
'iron' becomes 'Fe' (chemical symbol for iron).
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'river' becomes 'tyne' (river in the North East of England).
'fe'+'tyne'='fetyne'
'sat' going around 'fetyne' is 'SAFETY-NET'.
Can you help me to learn more?
(Other definitions for safety net that I've seen before include "It catches acrobat in case of fall" , "One stops diver" , "Guarantee of security" , "protective device" , "This may prevent dangerous mishap" .)