Liking forces engaged initially in battle (9)
I believe the answer is:
affection
'liking' is the definition.
(I've seen this before)
'forces engaged initially in battle' is the wordplay.
'forces' becomes 'ff' (I've seen this in other clues).
'initially' indicates taking the first letters.
'in' is an insertion indicator.
'battle' becomes 'action' (military action).
The first letter of 'engaged' is 'e'.
'ff'+'e'='ffe'
'ffe' inserted into 'action' is 'AFFECTION'.
(Other definitions for affection that I've seen before include "Feeling of love or strong liking" , "Feeling of fondness" , "Fondness, tenderness" , "Warm regard" , "Goodwill" .)