It's no good harbouring a worry constantly (3)
I believe the answer is:
nag
'worry constantly' is the definition.
(I've seen this before)
'it's no good harbouring a' is the wordplay.
'it's no' becomes 'n' (common abbreviation**).
'good' becomes 'g' (abbreviation).
'harbouring' is an insertion indicator.
'n'+'g'='ng'
'ng' enclosing 'a' is 'NAG'.
(Other definitions for nag that I've seen before include "(Of pain) ache dully; horse (colloq.)" , "Horse; keep scolding" , "Complain, worry" , "Scold; inferior horse" , "plague" .)