Line up outside initially under pressure (6)
I believe the answer is:
duress
'pressure' is the definition.
(I've seen this before)
'line up outside initially under' is the wordplay.
'line up' becomes 'dress' (synonyms).
'outside' is an insertion indicator (some letters go outside others).
'initially under' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dress' enclosing 'u' is 'DURESS'.
Can you help me to learn more?
(Other definitions for duress that I've seen before include "Hardship" , "Use of force or threats, often vile" , "The use of force or threats to get one's way" , "Pressure applied by druses" , "Compulsory force or threat" .)