Merry GI keeps company with sergeant at the front (6)
I believe the answer is:
jocose
'merry' is the definition.
(I've seen this in another clue)
'gi keeps company with sergeant at the front' is the wordplay.
'gi' becomes 'joe' (I can't explain this - if you can you should give a lot more credence to this answer).
'keeps' is an insertion indicator (inserted letters are kept inside).
'company' becomes 'co' (abbreviation).
'with' says to put letters next to each other.
'at the front' says to take the initial letters.
The initial letter of 'sergeant' is 's'.
'co'+'s'='cos'
'joe' placed around 'cos' is 'JOCOSE'.
Can you help me to learn more?
(Other definitions for jocose that I've seen before include "Jesting and playful" , "Playful and joking" , "Merry" , "Jesting, humourous" , "Playful, making jokes" .)