... significant moment defending second display of force (3,5)
I believe the answer is:
big stick
'display of force' is the definition.
The definition suggests a singular noun which matches the answer.
'significant moment defending second' is the wordplay.
'significant' becomes 'big' ('big' can be similar in meaning to 'significant').
'moment' becomes 'tick' (I've seen this before**).
'defending' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'big'+'tick'='bigtick'
'bigtick' enclosing 's' is 'BIG STICK'.
Can you help me to learn more?
(Other definitions for big stick that I've seen before include "it's worse than carrot!" , "Informal threat of force - using cudgel?" .)