Theatre boxes always cheapest way to go? (8)
I believe the answer is:
steerage
'cheapest way to go?' is the definition.
(cheapest accommodation on a ship)
'theatre boxes always' is the wordplay.
'theatre' becomes 'stage' (stage is a kind of theatre).
'boxes' means one lot of letters goes inside another (the outside letters must 'box' inside letters in).
'always' becomes 'eer' (poetic way of saying ever).
'stage' going around 'eer' is 'STEERAGE'.
(Other definitions for steerage that I've seen before include "Cheap part of ship" , "avoided by the "posh" set" , "Cheapest classes" , "Cheap seats" , "space on vessel" .)