Understand leader of fascists, in briefly (6)
I believe the answer is:
fathom
'understand' is the definition.
(to fathom something is to understand it)
'leader of fascists in briefly' is the wordplay.
'leader of' indicates taking the first letters.
'in' becomes 'at home' (someone at home is 'in').
'briefly' means to remove the last letter.
The initial letter of 'fascists' is 'f'.
'athome' with its last letter taken away is 'athom'.
'f'+'athom'='FATHOM'
(Other definitions for fathom that I've seen before include "It's six feet deep" , "Unit of depth - comprehend it" , "Measure of depth, 6 ft." , "Work out" , "Sea depth of six feet" .)