Get gripes regularly: content's missing from supermarket with most small branches (9)

'with most small branches' is the definition.
(twigs are small branches)

'get gripes regularly content's missing from supermarket' is the wordplay.
'get' becomes 'twig' (both can mean to understand or realise).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'content's missing from' suggests removing the centre (the letters contained by the word are removed).
'supermarket' with its middle removed is 'st'.
The alternating letters of 'gripes' are 'gie'.
'twig'+'gie'+'st'='TWIGGIEST'