Underground I'm better, not seen regularly (5)
I believe the answer is:
metro
'underground' is the definition.
(synonyms)
'i'm better not seen regularly' is the wordplay.
'seen regularly' means one should take alternating letters.
'im'+'better'+'not'='imbetternot'
The alternating letters of 'imbetternot' are 'METRO'.
(Other definitions for metro that I've seen before include "Urban railway system" , "what moves many Parisians" , "Parisian underground railway" , "Parisian rail system" , "The underground railway system in Paris" .)