Old boy isn't honest about German forces (7)
I believe the answer is:
obliges
'forces' is the definition.
(I've seen this before)
'old boy isn't honest about german' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'isn't honest' becomes 'lies' (I've seen this in another clue).
'about' is an insertion indicator.
'german' becomes 'g' (abbreviation).
'lies' placed around 'g' is 'liges'.
'ob'+'liges'='OBLIGES'
(Other definitions for obliges that I've seen before include "Accommodates" , "Compels" , "Forces" .)