Alarm used by father, mostly pleasant (5)
I believe the answer is:
panic
'alarm' is the definition.
(alarm can mean panic or worry)
'by father mostly pleasant' is the wordplay.
'by' says to put letters next to each other.
'father' becomes 'pa' (pa can mean dad or father).
'mostly' means to remove the last letter.
'pleasant' becomes 'nice' (associated in meaning).
'nice' with its last letter removed is 'nic'.
'pa'+'nic' is 'PANIC'.
'used' acts as a link.
(Other definitions for panic that I've seen before include "Sudden uncontrollable fear" , "Widespread fear" , "much unease" , "Take fright, lose the head" , "Alarm, scare" .)