Swimmer, about 90, is to show great skill (5)
I believe the answer is:
excel
'to show great skill' is the definition.
I can't tell whether this defines the answer.
'swimmer about 90' is the wordplay.
'swimmer' becomes 'eel' (eel is a type of fish).
'about' indicates putting letters inside.
'90' becomes 'xc' (Roman numerals).
'eel' going around 'xc' is 'EXCEL'.
'is' is the link.
Can you help me to learn more?
(Other definitions for excel that I've seen before include "well above average" , "Surpass; shine" , "Be pre-eminent" , "Do really well, outshine others" , "Do superbly well" .)