Regiment's first to penetrate enemy front (4)
I believe the answer is:
fore
'front' is the definition.
(fore is a kind of front)
'regiment's first to penetrate enemy' is the wordplay.
'first' says to take the initial letters.
'to penetrate' indicates putting letters inside.
'enemy' becomes 'foe' (I've seen this before).
The first letter of 'regiments' is 'r'.
'r' going inside 'foe' is 'FORE'.
(Other definitions for fore that I've seen before include "Golfer's warning shout" , "Placed in front" , "Warning cry of golfer" , "Word of warning to golfers" , "Ancestor" .)