In disguise, he'd creep in sneakily (10)
I believe the answer is:
enciphered
'in disguise' is the definition.
Although both the answer and 'disguise' are verbs, they are not in the same form of the verb.
'he'd creep in sneakily' is the wordplay.
'sneakily' indicates anagramming the letters (a tricksy or sneaky ordering of the letters).
'hed'+'creep'+'in'='hedcreepin'
'hedcreepin' anagrammed gives 'ENCIPHERED'.
Can you help me to learn more?
(Another definition for enciphered that I've seen is " were scrambling?".)