Father, mostly friendly, using alarm (5)
I believe the answer is:
panic
'alarm' is the definition.
(alarm can mean panic or worry)
'father mostly friendly' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'mostly' means to remove the last letter (most but not all of the word).
'friendly' becomes 'nice' (nice can mean kind or friendly).
'nice' with its final letter taken off is 'nic'.
'pa'+'nic'='PANIC'
'using' is the link.
(Other definitions for panic that I've seen before include "Contagious fear" , "Sudden overpowering anxiety" , "Overwhelming terror" , "displaying great fear" , "Take fright, lose the head" .)