Hold fast? He can in trouble! (7)
I believe the answer is:
enchain
'hold fast?' is the definition.
(I've seen this before)
'he can in trouble' is the wordplay.
'trouble' indicates an anagram.
'he'+'can'+'in'='hecanin'
'hecanin' with letters rearranged gives 'ENCHAIN'.
(Other definitions for enchain that I've seen before include "Bind" , "Secure" , "Immobilise" , "Shackle" , "Make fast" .)