Unknown outsiders joined because it's not secure (6)
I believe the answer is:
untied
'because it's not secure' is the definition.
I can't judge whether this definition defines the answer.
'unknown outsiders joined' is the wordplay.
'outsiders' means to remove the middle letters (only the letters on the outside of the word remain).
'joined' becomes 'tied' (I've seen this in another clue).
'unknown' with its middle removed is 'un'.
'un'+'tied'='UNTIED'
Can you help me to learn more?
(Other definitions for untied that I've seen before include "Loosened" , "Released from 19" , "Undid the knot" , "Disconnected" , "No longer bound" .)