Fliers must be strict occasionally with computer things (7)
I believe the answer is:
titmice
'fliers' is the definition.
(titmouse is a kind of bird)
'strict occasionally with computer things' is the wordplay.
'occasionally' means one should take alternating letters.
'with' says to put letters next to each other.
'computer things' becomes 'mice' (I am not sure about this - if you are sure you should believe this answer much more).
The alternate letters of 'strict' are 'tit'.
'tit'+'mice'='TITMICE'
'must be' is the link.
Can you help me to learn more?
(Other definitions for titmice that I've seen before include "Small, active songbirds" , "Small birds" , "little birds" , "Little singers" .)