Fahrenheit, in short, is a unit (6)
I believe the answer is:
fathom
'unit' is the definition.
(fathom is a kind of unit)
'fahrenheit in short' is the wordplay.
'fahrenheit' becomes 'f' (abbreviation).
'in' becomes 'athome' ('at home' can be similar in meaning to 'in').
'short' means to remove the last letter.
'athome' with its final letter taken off is 'athom'.
'f'+'athom'='FATHOM'
'is a' is the link.
(Other definitions for fathom that I've seen before include "''Full ... five, thy father lies'' (The Tempest)" , "Work out" , "Unit of six feet in measuring depth of water" , "Come to understand - unit of depth" , "Discover the meaning of - six feet" .)