Transgressor, not the first to get inside (5)
I believe the answer is:
inner
'inside' is the definition.
('inner' can be similar in meaning to 'inside')
'transgressor not the first' is the wordplay.
'transgressor' becomes 'sinner' (I've seen this before).
'not the first' means to remove the first letter.
'sinner' with its initial letter removed is 'INNER'.
'to get' is the link.
(Other definitions for inner that I've seen before include "Located or occurring closer to a centre" , "- - sanctum" , "Central, internal" , "Target next to bull" , "pretty good shot" .)