Period working out in southern lakes (5)
I believe the answer is:
spell
'period' is the definition.
(spell is a kind of period)
'working out in southern lakes' is the wordplay.
'working out' becomes 'pe' (physical education).
'in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'lakes' becomes 'll' (I've seen this in other clues).
's'+'ll'='sll'
'pe' inserted within 'sll' is 'SPELL'.
(Other definitions for spell that I've seen before include "Charm; short period" , "Incantation; write in letters" , "influence" , "Name in order the letters of the alphabet involved" , "Give out letters" .)