Colourful shirt showing style - Kelvin investing in a couple (7)
I believe the answer is:
dashiki
'colourful shirt' is the definition.
'style kelvin investing in a couple' is the wordplay.
'style' becomes 'dash' ('dash' can be a synonym of 'style').
'kelvin' becomes 'k' ('K' can be a synonym of 'kelvin'**).
'investing in' means one lot of letters goes inside another.
'a couple' becomes 'ii' (I've seen this in other clues).
'k' placed into 'ii' is 'iki'.
'dash'+'iki'='DASHIKI'
'showing' is the link.
(Another definition for dashiki that I've seen is " Colourful African shirt".)