Mean to force Nigel to admit the old boy (7)
I believe the answer is:
ignoble
'mean' is the definition.
(similar in meaning)
'force nigel to admit the old boy' is the wordplay.
'force' is an anagram indicator (letters must be forced into a new order).
'to admit' is an insertion indicator.
'the old boy' becomes 'ob' (abbreviation).
'nigel' with letters rearranged gives 'ignle'.
'ignle' enclosing 'ob' is 'IGNOBLE'.
'to' acts as a link.
(Other definitions for ignoble that I've seen before include "'Far from the madding crowd's . . . . . . . strife'" , "'Base, dishonourable (7)'" , "Completely lacking in character or purpose" , "No bilge! (anag)" , "Mean - of low birth" .)