Idiot blocking American military groups? (5)
I believe the answer is:
units
'military groups?' is the definition.
(I've seen this in another clue)
'idiot blocking american' is the wordplay.
'idiot' becomes 'nit' (nit can mean a stupid person).
'blocking' indicates putting letters inside (block can mean to plug a gap in something).
'american' becomes 'US' (United States).
'nit' placed inside 'us' is 'UNITS'.
(Other definitions for units that I've seen before include "Shops in a mall, for example" , "Single parts of a whole" , "Eg, cm, kg and ft" , "Modules" , "Measures of alcohol" .)