Gents swimming across river on Thursday, showing talent (8)
I believe the answer is:
strength
'talent' is the definition.
(thesaurus)
'gents swimming across river on thursday' is the wordplay.
'swimming' indicates an anagram.
'across' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'on' says to put letters next to each other.
'thursday' becomes 'th' (common abbreviation).
'gents' with letters rearranged gives 'steng'.
'steng' placed around 'r' is 'streng'.
'streng'+'th'='STRENGTH'
'showing' is the link.
(Other definitions for strength that I've seen before include "Physical or mental power" , "The longest one-syllable word in English - might" , "Vigour, potency" , "Beneficial attribute" , "Physical energy" .)