Buoyant United lead, with worry to follow (6)
I believe the answer is:
upbeat
'buoyant' is the definition.
(thesaurus)
'united lead with worry to follow' is the wordplay.
'united' becomes 'u' (as in Man U, English football team).
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'with' means one lot of letters go next to another.
'worry' becomes 'eat' (something worrying may eat someone up).
'to follow' says to put letters next to each other (some letters go after or follow others).
'u'+'pb'+'eat'='UPBEAT'
(Other definitions for upbeat that I've seen before include "Promising" , "Relentlessly optimistic" , "signal from sergeant, say, at end of bar?" , "Cheerful and sanguine - as in music?" , "bright and cheery" .)