Slump, having a breather during workout (6)
I believe the answer is:
plunge
'slump' is the definition.
(thesaurus)
'a breather during workout' is the wordplay.
'a breather' becomes 'lung' (I've seen this in another clue).
'during' means one lot of letters goes inside another.
'workout' becomes 'pe' (I am not sure about this - if you are sure you should believe this answer much more).
'lung' placed inside 'pe' is 'PLUNGE'.
'having' acts as a link.
Can you help me to learn more?
(Other definitions for plunge that I've seen before include "'Dive, plummet (6)'" , "Nose-dive" , "Pitch" , "A steep drop" , "Dive quickly" .)