Strength shown by triathlete initially in front (5)
I believe the answer is:
forte
'strength' is the definition.
(synonyms)
'triathlete initially in front' is the wordplay.
'initially' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'front' becomes 'fore' (associated in meaning).
The initial letter of 'triathlete' is 't'.
't' placed into 'fore' is 'FORTE'.
'shown by' is the link.
(Other definitions for forte that I've seen before include "Musically, loud" , "Loud (mus.)" , "sort of musical passage" , "Skill" , "Thing at which one excels" .)