Cracked it, entering most of interior (6)
I believe the answer is:
insane
'cracked' is the definition.
(similar in meaning)
'it entering most of interior' is the wordplay.
'it' becomes 'sa' (SA can mean sex appeal, as in 'it').
'entering' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'interior' becomes 'inner' (synonyms).
'inner' with its final letter removed is 'inne'.
'sa' inserted within 'inne' is 'INSANE'.
(Other definitions for insane that I've seen before include "Out of one's mind" , "having a screw loose" , "Round the bend" , "Eg, gun or knife" , "Irresponsible" .)