A meaning that's lost (6)
I believe the answer is:
adrift
'lost' is the definition.
(thesaurus)
'a meaning' is the wordplay.
'meaning' becomes 'drift' (drift is a kind of meaning).
'a'+'drift'='ADRIFT'
'that's' acts as a link.
(Other definitions for adrift that I've seen before include "like Mary Celeste?" , "'Directionless, floating freely (6)'" , "(Of ship) out of control" , "Unsettled or unanchored" , "getting nowhere?" .)