Commander on land inspiring serious fear (13)
I believe the answer is:
consternation
'fear' is the definition.
(I know that consternation is a type of fear)
'commander on land inspiring serious' is the wordplay.
'commander on land' becomes 'conation' (I can't justify this - if you can you should give a lot more credence to this answer).
'inspiring' means one lot of letters goes inside another.
'serious' becomes 'stern' (I've seen this in another clue).
'conation' placed around 'stern' is 'CONSTERNATION'.
(Other definitions for consternation that I've seen before include "Feelings of dismay" , "Dismay - cast-iron tenon (anag)" , "Anxiety, dismay" , "Alarm" , "Worry" .)