Grasp plump, hot, old male (6)
I believe the answer is:
fathom
'grasp' is the definition.
(I've seen this in another clue)
'plump hot old male' is the wordplay.
'plump' becomes 'fat' (similar in meaning).
'hot' becomes 'h' (abbreviation eg on taps).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'male' becomes 'm' (common abbreviation).
'fat'+'h'+'o'+'m'='FATHOM'
(Other definitions for fathom that I've seen before include "Sea depth of six feet" , "Come to understand - unit of depth" , "Six feet; make sense of" , "Nautical depth of six feet" , "''Full ... five, thy father lies'' (The Tempest)" .)