Travelled in formal wear for a decade (8)
I believe the answer is:
twenties
'for a decade' is the definition.
(eg the 1920s)
'travelled in formal wear' is the wordplay.
'travelled' becomes 'went' (I've seen this before).
'in' indicates putting letters inside.
'formal wear' becomes 'ties' (ties are worn formally).
'went' going within 'ties' is 'TWENTIES'.
(Other definitions for twenties that I've seen before include "Scores" , "ten years" , "what lies ahead for 27 [TEENAGER]?" , "The decade was said to be roaring" , "First adult years" .)