Comparatively muscular male joins row over female (7)
I believe the answer is:
heftier
'comparatively muscular' is the definition.
('hefty' can be a synonym of 'muscular')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for heftier that I've seen before include "more vigorous" , "Bigger" , "More weighty" .)