…as he escaped hollow under seat? (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'escaped hollow under seat?' is the wordplay.
'escaped hollow' becomes 'nic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'under' means one lot of letters go next to another.
'seat?' becomes 'arse' (synonyms).
'nic' put after 'arse' is 'ARSENIC'.
'he' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Element No 33" , ", it goes with old lace" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "killer" , "Brittle grey-green semi-metal, atomic number 33" .)