Your bad swimming involves constant rigidity (8)
I believe the answer is:
obduracy
'rigidity' is the definition.
(thesaurus)
'your bad swimming involves constant' is the wordplay.
'swimming' indicates an anagram.
'involves' is an insertion indicator.
'constant' becomes 'c' (mathematical abbreviation).
'your'+'bad'='yourbad'
'yourbad' with letters rearranged gives 'obduray'.
'obduray' going around 'c' is 'OBDURACY'.
(Other definitions for obduracy that I've seen before include "Stubbornness" , "Refusal to give in" , "Mulishness" .)