Main restriction almost absent in trade (3,4)
I believe the answer is:
sea wall
'main restriction' is the definition.
The definition suggests a singular noun which matches the answer.
'almost absent in trade' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'absent' becomes 'away' (similar in meaning).
'in' is an insertion indicator.
'trade' becomes 'sell' (to sell is to trade something for money).
'away' with its last letter taken away is 'awa'.
'awa' put inside 'sell' is 'SEA-WALL'.
Can you help me to learn more?
(Other definitions for sea wall that I've seen before include "Land erosion protector" , "division appearing in waves" , "Embankment serving as a breakwater" , "It checks main" , "Coastal construction" .)