Anniversary — then tie-in needs developing (9)
I believe the answer is:
ninetieth
'anniversary' is the definition.
(ninetieth anniversary)
'then tie-in needs developing' is the wordplay.
'needs developing' indicates an anagram.
'then'+'tie'+'in'='thentiein'
'thentiein' with letters rearranged gives 'NINETIETH'.
(Other definitions for ninetieth that I've seen before include "Birthday" , "ordinal" , "in the last minute?" , "An anniversary" .)