Dance in the morning to charge muscles up (5)
I believe the answer is:
samba
'dance' is the definition.
(Brazilian dance)
'morning to charge muscles up' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'to charge' indicates putting letters inside (charge can mean to fill).
'muscles' becomes 'abs' (short for abdominals).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'abs' in reverse letter order is 'sba'.
'am' put within 'sba' is 'SAMBA'.
'in the' is the link.
(Other definitions for samba that I've seen before include "Lively ballroom dance from Brazil" , "moves from Brazil" , "Brazilian dance" , "South American dance" , "Latin dance" .)