Second person once with husband out cycling (4)
I believe the answer is:
thou
'second person once' is the definition.
(historical second person pronoun)
'with husband out cycling' is the wordplay.
'with' indicates putting letters inside.
'husband' becomes 'h' (abbreviation).
'cycling' is an anagram indicator.
'out' anagrammed gives 'tou'.
'h' put inside 'tou' is 'THOU'.
(Other definitions for thou that I've seen before include "once paired with art" , "Biblical 'you'" , "You, poetically of old" , "Old-fashioned pronoun" , "You (archaic, but still used in prayers)" .)