Cycling mostly with Matilda in minimal attire (6)
I believe the answer is:
bikini
'minimal attire' is the definition.
(type of swimwear)
'cycling mostly with matilda' is the wordplay.
'cycling' becomes 'biking' ('bike' can be a synonym of 'cycle').
'mostly' means to remove the last letter.
'with' is a charade indicator (letters next to each other).
'matilda' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'biking' with its last letter taken off is 'bikin'.
'bikin'+'i'='BIKINI'
'in' acts as a link.
(Other definitions for bikini that I've seen before include "providing little cover for swimmers" , "Small two-piece bathing costume" , "Skimpy two-piece swimsuit" , "Atoll and swimming costume" , "Clothing to wear on the Pacific atoll?" .)