Barring funny regulars bringing up the Spanish ship (6)
I believe the answer is:
unless
'barring' is the definition.
(thesaurus)
'funny regulars bringing up the spanish ship' is the wordplay.
'regulars' means one should take alternating letters (letters selected at regular intervals).
'bringing up' says the letters should be written in reverse (I've seen 'brought up' mean this (in a down clue, letters go up)).
'the spanish' becomes 'el' ('the' in Spanish).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
The alternating letters of 'funny' are 'un'.
'el' written backwards gives 'le'.
'un'+'le'+'ss'='UNLESS'
(Other definitions for unless that I've seen before include "Conjunction meaning except when" , "Except on the condition that" , "Except, save" , "Otherwise than if" , "Conditional on" .)