Before opening old tub stuff oneself (7)
I believe the answer is:
overeat
'stuff oneself' is the definition.
(I've seen this before)
'before opening old tub' is the wordplay.
'before' becomes 'ere' (poetic term for 'before').
'opening' is an insertion indicator (some letters open a gap in others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tub' becomes 'vat' (synonyms).
'o'+'vat'='ovat'
'ere' inserted into 'ovat' is 'OVEREAT'.
(Other definitions for overeat that I've seen before include "ignore threat of a big corporation?" , "Stuff oneself silly" , "Be gluttonous" , "Gorge oneself" , "Consume too much food, stuff your stomach" .)