Worrying at first, a phase in degeneration (7)
I believe the answer is:
wastage
'degeneration' is the definition.
(degenerating or wasting away)
'worrying at first a phase' is the wordplay.
'at first' indicates taking the first letters.
'phase' becomes 'stage' (synonyms).
The first letter of 'worrying' is 'w'.
'w'+'a'+'stage'='WASTAGE'
'in' is the link.
(Other definitions for wastage that I've seen before include "Loss through careless use" , "Unnecessary loss" , "Useless spending" , "Loss (if natural, without dismissals)" , "Atrophy -- careless destruction" .)