Type of soldiers covering partners with return fire (4,5)
I believe the answer is:
sans serif
'type' is the definition.
(sans serif is a kind of type)
'soldiers covering partners with return fire' is the wordplay.
'soldiers' becomes 'sas' (Special Air Service).
'covering' indicates putting letters inside.
'partners' becomes 'ns' (North and South are partners in the game bridge).
'with' means one lot of letters go next to another.
'return' says the letters should be written in reverse.
'fire' reversed gives 'erif'.
'sas' placed around 'ns' is 'sanss'.
'sanss'+'erif'='SANS SERIF'
'of' acts as a link.
(Other definitions for sans serif that I've seen before include "Style of typeface" , "Plain typeface" , "Plain style of type" , "plain type of face" , "Style of font" .)