Occasionally overlooked illegal parking in spring (4)
I believe the answer is:
leap
'spring' is the definition.
(synonyms)
'occasionally overlooked illegal parking' is the wordplay.
'occasionally overlooked' indicates alternate letters (letters removed at regular intervals).
'parking' becomes 'p'.
The alternate letters of 'illegal' are 'lea'.
'lea'+'p'='LEAP'
'in' acts as a link.
(Other definitions for leap that I've seen before include "Jump a long way" , "Jump across" , "identify extra day?" , "Jump, spring" , "Sudden surge" .)