Time of year car park is regularly close to full (5)
I believe the answer is:
april
'time of year' is the definition.
'car park is regularly close to full' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'close to' indicates one should take the final letters (the close/ending of).
The last letter of 'full' is 'l'.
'car'+'park'+'is'='carparkis'
The alternate letters of 'carparkis' are 'apri'.
'apri'+'l'='APRIL'
(Other definitions for april that I've seen before include "TS Eliot's 'cruelest month'" , "Calendar month" , "All in the ..... evening (song)" , "Woman" , "Traditionally showery month" .)