Sierra regularly parking under branch (5)
I believe the answer is:
sprig
'branch' is the definition.
(I know that sprig is a type of branch)
'sierra regularly parking under' is the wordplay.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'under' says to put letters next to each other (in a down clue, some letters go below others).
The alternating letters of 'parking' are 'prig'.
's'+'prig'='SPRIG'
(Other definitions for sprig that I've seen before include "Youth" , "A tack; a little shoot" , "Tiny branch" , "piece of holly" , "Headless nail; leafy ornament" .)