The one inside cannot possibly hold it (7)
I believe the answer is:
contain
'the one inside cannot possibly hold it' is the definition.
I can't judge whether this definition defines the answer.
'the one inside cannot possibly' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'inside' indicates putting letters inside.
'possibly' indicates an anagram.
'cannot' is an anagram of 'contan'.
'i' inserted inside 'contan' is 'CONTAIN'.
Can you help me to learn more?
(Other definitions for contain that I've seen before include "Include - restrict" , "Control, restrain" , "I cannot (anag) - hold back" , "Accommodate" , "keep within bounds" .)