Is first to be locked in the big prison (5)
I believe the answer is:
opens
'is first to be locked' is the definition.
I can't judge whether this defines the answer.
'in the big prison' is the wordplay.
'in' is an insertion indicator.
'the big' becomes 'os'.
'prison' becomes 'pen' (I've seen this before).
'os' going around 'pen' is 'OPENS'.
Can you help me to learn more?
(Other definitions for opens that I've seen before include "Allows access" , "releases" , "sports competitions" , "competitions welcoming all-comers" , "Provides access to" .)